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Luminiferous aether

2007 Schools Wikipedia Selection. Related subjects: Space (Astronomy)

   The luminiferous aether: it was hypothesised that the Earth moves
   through a "medium" of aether that carries light
   Enlarge
   The luminiferous aether: it was hypothesised that the Earth moves
   through a "medium" of aether that carries light

   In the late 19th century luminiferous aether ("light-bearing aether")
   was the term used to describe a medium for the propagation of light.
   Later theories including special relativity were formulated without the
   aether concept, and today the aether is considered to be a superseded
   scientific theory.

   The word "aether" stems via Latin from the Greek αἰθήρ, from a root
   meaning "to kindle/burn/shine", which signified the substance thought
   in ancient times to fill the upper regions of space, beyond the clouds.

The history of light and aether

   Isaac Newton had assumed that light was made up of numerous small
   particles, in order to explain features such as its ability to travel
   in straight lines and reflect off surfaces. This theory was known to
   have its problems; although it explained reflection well, its
   explanation of refraction and diffraction was less pleasing. In order
   to explain refraction, in fact, Newton's Opticks (1704) postulated an
   "Aethereal Medium" transmitting vibrations faster than light, by which
   light (when overtaken) is put into "Fits of easy Reflexion and easy
   Transmission" (causing refraction and diffraction). Newton believed
   that these vibrations were related to things like heat radiation,
   saying:

          Is not the Heat of the warm Room convey'd through the Vacuum by
          the Vibrations of a much subtiler Medium than Air, which after
          the Air was drawn out remained in the Vacuum? And is not this
          Medium the same with that Medium by which Light is refracted and
          reflected, and by whose Vibrations Light communicates Heat to
          Bodies, and is put into Fits of easy Reflexion and easy
          Transmission?

   The modern understanding, of course, is that heat radiation is light,
   but Newton considered them two different phenomena (believing heat
   vibrations to be excited "when a Ray of Light falls upon the Surface of
   any pellucid Body"). He wrote that "I do not know what this Aether is",
   but that if it consists of particles then they must be "exceedingly
   smaller than those of Air, or even than those of Light: The exceeding
   smallness of its Particles may contribute to the greatness of the force
   by which those Particles may recede from one another, and thereby make
   that Medium exceedingly more rare and elastick than Air, and by
   consequence exceedingly less able to resist the motions of Projectiles,
   and exceedingly more able to press upon gross Bodies, by endeavoring to
   expand itself."

   Christiaan Huygens, prior to Newton, had hypothesized that light itself
   was a wave propagating through an Aether, but Newton rejected this
   idea. The main reason for his rejection stemmed from the fact that both
   men could apparently only envision light to be a longitudinal wave,
   like sound and other mechanical waves in gases and fluids. However,
   longitudinal waves by necessity have only one form for a given
   propagation direction, rather than two polarizations as in a transverse
   wave, and thus they were unable to explain the phenomenon of
   birefringence (where two polarizations of light are refracted
   differently by a crystal). Instead, Newton preferred to imagine
   non-spherical particles (or "corpuscles") of light with different
   "sides" that give rise to birefringence. A further reason why Newton
   rejected light as waves in a medium, however, was because such a medium
   would have to extend everywhere in space, and would thereby "disturb
   and retard the Motions of those great Bodies" (the planets and comets)
   and thus "as it [light's medium] is of no use, and hinders the
   Operation of Nature, and makes her languish, so there is no evidence
   for its Existence, and therefore it ought to be rejected."

   In 1720 James Bradley carried out a series of experiments attempting to
   measure stellar parallax. Although he failed to detect any parallax
   (thereby placing a lower limit on the distance to stars), he discovered
   another effect, stellar aberration, an effect which depends not on
   position (as in parallax), but on speed. He noticed that the apparent
   position of the star changed as the Earth moved around its orbit.
   Bradley explained this effect in the context of Newton's corpuscular
   theory of light, by showing that the aberration angle was given by
   simple vector addition of the Earth's orbital velocity and the velocity
   of the corpuscles of light (just as vertically falling raindrops strike
   a moving object at an angle). Knowing the Earth's velocity and the
   aberration angle, this enabled him to estimate the speed of light. To
   explain stellar aberration in the context of an ether-based theory of
   light was regarded as more problematic, because it requires that the
   ether be stationary even as the Earth moves through it – precisely the
   problem that led Newton to reject a wave model in the first place.

   However, a century later, Young and Fresnel revived the wave theory of
   light when they pointed out that light could be a transverse wave
   rather than a longitudinal wave—the polarization of a transverse wave
   (like Newton's "sides" of light) could explain birefringence, and in
   the wake of a series of experiments on diffraction the particle model
   of Newton was finally abandoned. Physicists still assumed, however,
   that like mechanical waves, light waves required a medium for
   propagation, and thus required Huygens' idea of an aether "gas"
   permeating all space.

   However a transverse wave apparently required the propagating medium to
   behave as a solid, as opposed to a gas or fluid. The idea of a solid
   that did not interact with other matter seemed a bit odd, and
   Augustin-Louis Cauchy suggested that perhaps there was some sort of
   "dragging", or "entrainment", but this made the aberration measurements
   difficult to understand. He also suggested that the absence of
   longitudinal waves suggested that the aether had negative
   compressibility; but George Green pointed out that such a fluid would
   be unstable. George Gabriel Stokes became a champion of the entrainment
   interpretation, developing a model in which the aether might be (by
   analogy with pine pitch) rigid at very high frequencies and fluid at
   lower speeds. Thus the Earth could move through it fairly freely, but
   it would be rigid enough to support light.

   Later, Maxwell's equations showed that light is an electromagnetic
   wave. Maxwell's equations required that all electromagnetic waves in
   vacuum propagate at a fixed speed, c. As this can only occur in one
   reference frame in Newtonian physics (see Galilean-Newtonian
   relativity), the aether was hypothesized as the absolute and unique
   frame of reference in which Maxwell's equations hold. That is, the
   aether must be "still" universally, otherwise c would vary from place
   to place. Maxwell himself proposed several mechanical models of aether
   based on wheels and gears and George FitzGerald even constructed a
   working model of one of them. These models were non-trivial especially
   because they had to agree with the fact that the electromagnetic waves
   are transverse but never longitudinal.

   Nevertheless, by this point the mechanical qualities of the aether had
   become more and more magical: it had to be a fluid in order to fill
   space, but one that was millions of times more rigid than steel in
   order to support the high frequencies of light waves. It also had to be
   massless and without viscosity, otherwise it would visibly affect the
   orbits of planets. Additionally it appeared it had to be completely
   transparent, non-dispersive, incompressible, and continuous at a very
   small scale.

   Contemporary scientists were aware of the problems, but aether theory
   was so entrenched in physical law by this point that it was simply
   assumed to exist. In 1908 Oliver Lodge gave a speech in behalf of Lord
   Rayleigh to the Royal Institution on this topic, in which he outlined
   its physical properties, and then attempted to offer reasons why they
   were not impossible. Nevertheless he was also aware of the criticisms,
   and quoted Lord Salisbury as saying that "aether is little more than a
   nominative case of the verb to undulate". Others criticized it as an
   "English invention", although Rayleigh jokingly corrected them to state
   it was actually an invention of the Royal Institution.

   By the early 20th Century, aether theory was in trouble: A series of
   increasingly complex experiments had been carried out in the late 1800s
   to try 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. Lorentz and
   Fitzgerald offered a more elegant solution to how the motion of an
   absolute aether could be undetectable (length contraction), but if
   their equations were correct, the new special theory of relativity
   (1905) could generate the same mathematics without referring to an
   aether at all. Aether fell to Occam's Razor.

Aether and classical mechanics

   The key difficulty with the aether hypothesis arose from the
   juxtaposition of the two well-established theories of Newtonian
   dynamics and Maxwell's electromagnetism. Under a Galilean
   transformation the equations of Newtonian dynamics are invariant,
   whereas those of electromagnetism are not. Basically this means that
   while physics should remain the same in non-accelerated experiments,
   light would not follow the same rules because it is travelling in the
   universal "aether frame". Some effect caused by this difference should
   be detectable.

   A simple example concerns the model on which aether was originally
   built: sound. The speed of propagation for mechanical waves, the speed
   of sound, is defined by the mechanical properties of the medium. For
   instance, if one is in an airliner, you can still carry on a
   conversation with the person beside you because the sound of your words
   are travelling along with the air inside the aircraft. This effect is
   basic to all Newtonian dynamics, which says that everything from sound
   to the trajectory of a thrown baseball should all remain the same in
   the aircraft as sitting "still" on the Earth. This is the basis of the
   Galilean transformation, and the concept of "frame of reference".

   But the same was not true for light. Since Maxwell's mathematics
   demanded a single, universal, speed for the propagation of light, based
   not on local conditions, but two measured properties that were assumed
   to be the same throughout the universe. If these numbers did change,
   there should be noticeable effects in the sky; stars in different
   directions would have different colors, for instance. Certainly they
   would remain constant within a small volume, inside the aircraft in our
   example for instance, which implies that light would not "follow along"
   with the aircraft (or the Earth) in a fashion similar to sound. Nor
   could light "change media", for instance, using the atmosphere while
   near the Earth. It had already been demonstrated that if this were so,
   the sky would be colored in different directions as the light moved
   from the still medium of the aether to the moving medium of the Earth's
   atmosphere, causing diffraction.

   Thus at any point there should be one special coordinate system, "at
   rest relative to the aether". Maxwell noted in the late 1870s that
   detecting motion relative to this aether should be easy enough – light
   travelling "along" with the motion of the Earth would have a different
   speed than light travelling "backward", as they would both be moving
   against the unmoving aether. Even if the aether had an overall
   universal flow, changes in position during the day/night cycle, or over
   the span of seasons, should allow the "drift" to be detected.

Experiments

   Numerous experiments were carried out in the late 1800s to test for
   this "aether wind" effect, but most were open to dispute due to low
   accuracy. Measurements on the speed of propagation were so inaccurate
   that comparing two speeds to look for a difference was essentially
   impossible.

   The famous Michelson-Morley experiment instead compared the source
   light with itself after being sent in different directions, looking for
   changes in phase in a manner that could be measured with extremely high
   accuracy. The publication of their result in 1887, the null result, was
   the first clear demonstration that something was seriously wrong with
   the "absolute aether" concept. A series of experiments using similar
   but increasingly sophisticated apparatus all returned the null result
   as well. A conceptually different experiment that also attempted to
   detect the motion of the aether was the 1903 Trouton-Noble experiment,
   which like Michelson-Morley obtained a null result.

   It is important to understand what "null result" means in this context.
   It does not mean there was no motion detected; rather it means that the
   results produced by the experiment were not compatible with the
   assumptions used to devise it. In this case the MM experiment showed a
   small positive velocity causing a movement of the fringing pattern of
   about 0.01 of a fringe; however it was too small to demonstrate the
   expected aether wind effect due to the earth's (seasonally varying)
   velocity which would have required a shift of 0.4 of a fringe, and the
   error was small enough that the value may have indeed been zero. More
   modern experiments have since reduced the possible value to a number
   very close to zero, about 10^-15.

   These "aether-wind" experiments led to its abandonment by some
   scientists, and to a flurry of efforts to "save" aether by assigning it
   ever more complex properties by others. Of particular interest was the
   possibility of "aether entrainment" or "aether drag", which would lower
   the magnitude of the measurement, perhaps enough to explain MMX
   results. However, as noted earlier, aether dragging already had
   problems of its own, notably aberration. A more direct measurement was
   made in the Hamar experiment, which ran a complete MM experiment with
   one of the "legs" placed between two massive lead blocks. If the aether
   was dragged by mass then this experiment would have been able to detect
   the drag caused by the lead, but again the null result was found.
   Similar experiments by Hoek placed one leg in a heavy vat of water. The
   theory was again modified, this time to suggest that the entrainment
   only worked for very large masses or those masses with large magnetic
   fields. This too was shown to be incorrect when Oliver Joseph Lodge
   noted no such effect around other planets.

   Another, completely different, attempt to save "absolute" aether was
   made in the Lorentz-Fitzgerald contraction hypothesis, which posited
   that everything was affected by travel through the aether. In this
   theory the reason the Michelson-Morley experiment "failed" was that it
   contracted in length in the direction of travel. That is, the light was
   being affected in the "natural" manner by its travel though the aether
   as predicted, but so was the experiment itself, cancelling out any
   difference when measured. Even Lorentz was not very happy with this
   suggestion, although it did neatly solve the problem and it was a first
   step towards relativity theory. Without referral to an ether, this
   physical interpretation of relativistic effects was shared by Kennedy
   and Thorndike in 1932 as they concluded that the interferometer's arm
   contracts and also the frequency of its light source "very nearly"
   varies in the way required by relativity.

   Another experiment purporting to show effects of an aether was Fizeau's
   1851 experimental confirmation of Fresnel's 1818 prediction that a
   medium with refractive index n moving with a velocity v would increase
   the speed of light traveling through the medium in the same direction
   as v from c/n to:

          \frac{c}{n} + \left( 1 - \frac{1}{n^2} \right) v

   That is, movement adds only a fraction of the medium's velocity to the
   light (predicted by Fresnel in order to make Snell's law work in all
   frames of reference, consistent with stellar aberration). This was
   initially interpreted to mean that the medium drags the aether along,
   with a portion of the medium's velocity, but that understanding was
   rejected after Wilhelm Veltmann demonstrated that the index n in
   Fresnel's formula depended upon the wavelength of light (so that the
   aether could not be moving at a wavelength-independent speed). With the
   advent of special relativity, Fresnel's equation was shown by Laue in
   1907 to be an approximation, valid for v much smaller than c, for the
   correct relativistic formula to add the velocities v (medium) and c/n
   (rest frame):

          \frac{c/n + v}{1 + \frac{v c/n} {c^2}} \approx \frac{c}{n} +
          \left( 1 - \frac{1}{n^2} \right) v +
          O\left(\frac{v^2}{c^2}\right).

   Variations on these themes continued for the next 30 years. Positive
   results were reported by several of the key researchers, including
   additional experiments by Michelson, Morley and Dayton Miller. Miller
   reported positive results on several occasions, but of a magnitude that
   required further modifications to the drag or contraction theories.
   During the 1920s a slew of increasingly accurate experiments returned
   the null result, and the positives were generally attributed to
   experimental errors.

   Other positive results included Sagnac in 1913, and the
   Michelson-Gale-Pearson experiment in 1925. This effect that is known as
   Sagnac effect is nowadays used in optical gyroscopes and shows that
   rotation is similarly "absolute" for light as it is for pendulums.
   Sagnac regarded this as evidence for the aether

End of aether?

   Aether theory was dealt another blow when the Galilean transformation
   and Newtonian dynamics were both modified by Albert Einstein's special
   theory of relativity, giving the mathematics of Lorentzian
   electrodynamics a new, "non-aether" context. Like most major shifts in
   scientific thought, the move away from aether theory did not happen
   immediately but, as experimental evidence built up, and as older
   scientists left the field and their places were taken by the young, the
   concept lost adherents.

   Einstein based his special theory on Lorentz's earlier work, but
   instead of suggesting that the mechanical properties of objects changed
   with their constant-velocity motion through an aether, he took the
   somewhat more radical step of suggesting that the math was a general
   transformation, and that the Galilean transformation was a "special
   case" that worked only at the low speeds we had studied up to that
   time. By applying the transformation to all inertial frames of
   reference, he demonstrated that physics remained invariant as it had
   with the Galilean transformation, but that light was now invariant as
   well.

   With the development of special relativity, the need to account for a
   single universal frame had disappeared -- and aether went along with
   it, or so it seemed.

   For Einstein the Lorentz transformation implied a radical conceptual
   change: that the concept of position in space or time was not absolute,
   but could differ depending on the observer's location and speed. This
   "oddness" of Einstein's interpretation led to special relativity being
   considered highly questionable for some time.

   All of this left the problem of light propagation through a vacuum.
   However, in another paper published the same month, Einstein also 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 to be simplifications of what is "really
   happening". 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"

   Lorentz on his side continued to use the aether concept. 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 reminded his audience of the
   fact 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".

   Paul Langevin was a strong supporter of special relativity but argued
   in 1911 that absolute effects from velocity change or acceleration
   (such as radiation) demonstrate the existence of an aether. As
   additional illustration he discussed the absolute effect of velocity
   change on time dilation on two space travelers. This example would
   later lead to the twin paradox.

   In the meantime Einstein changed his opinion about the aether concept.
   In a lecture meant for his inauguration at the University of Leiden in
   1920, Einstein stressed that space is "endowed with physical
   quantities" He held that general relativity attributed tangible
   physical properties to space including some kind of medium for light,
   although not a material one. Shortly before his lecture in Leyden in
   1920 he admitted in the paper: "Grundgedanken und Methoden der
   Relativitätstheorie in ihrer Entwicklung dargestellt":

   "Therefore I thought in 1905 that in physics one should not speak of
   the ether at all. This judgement was too radical though as we shall see
   with the next considerations about the general theory of relativity. It
   moreover remains, as before, allowed to assume a space-filling medium
   if one can refer to electromagnetic fields (and thus also for sure
   matter) as the condition thereof ".

   Also Michelson, who received the Nobel Prize in physics in 1907 for his
   optical studies, stated in 1923 that even if relativity is here to stay
   we don't have to reject the aether. Some other physicists who published
   their support for modern aether concepts were Herbert Ives, Paul Dirac
   and Geoffrey Builder.

   Ives was the first to positively measure the effect of speed on clock
   rates. He wrote in 1940 in a paper in Science:

   "I have considered the popular claim that the ether has been
   "abolished" [...]. Reverting to experimental findings I have reviewed
   the experiment of Sagnac, having in mind the claim that the ether can
   not be detected experimentally. I have asserted that, in the light of
   the experimentally found variation of clock rate with motion, this
   experiment does detect the ether."

   G. Builder asserted in a paper of 1958 that "there is therefore no
   alternative to the ether hypothesis" Professor Sherwin supported in
   1960 the "philosophical point of view" of Ives and Builder about the
   aether because of his own conclusion that clocks are "literally slowed
   down by the speed itself"

   Also Dirac stated in 1951 in an article in Nature, titled "Is there an
   ether?" that "we are rather forced to have an ether"

   The large majority of scientists disagreed with such views.

Continuing adherents

   Today, the majority of physicists hold that there is no need to imagine
   that a medium for light propagation exists. They believe that neither
   Einstein's general theory of relativity nor quantum mechanics have need
   for it and that there is no evidence for it. As such, a classical
   aether is an unnecessary addition to physics that violates the
   principle of Occam's razor.

   Moreover, it is hard to develop an aether theory that is consistent
   with all experiments of modern physics. Any new theory of aether must
   be consistent with all of the experiments testing phenomena of special
   relativity, general relativity, relativistic quantum mechanics, and so
   on. As outlined earlier, these conditions are often contradictory,
   making such a task inherently difficult.

   Nevertheless the intuitive appeal of a causal background for
   "relativistic" effects cannot be denied. Some physicists hold that
   there remain a number of problems in modern physics that are simplified
   by an aether concept, so that Occam's razor doesn't apply.

   A very small number of physicists (like Dayton Miller and Edward
   Morley) continued research on the aether for some time, and
   occasionally researchers such as Harold Aspden still promote the
   concept.

   A number of new aether concepts have been proposed in recent years.
   However, these aethers differ considerably from the classical
   luminiferous aether.

   In a controversial quantum approach to gravity called loop quantum
   gravity, spacetime is filled with a structure called the spin foam.
   Much like aether, it picks a privileged reference frame and is
   incompatible with Lorentz invariance, a symmetry of special theory of
   relativity. Its existence therefore potentially disagrees with the
   Michelson-Morley-like experiments.

   Maurizio Consoli of the Italian National Institute of Nuclear Physics
   in Catania, Sicily, argues in Physics Letters A (vol 333, p 355) that
   any Michelson-Morley type of experiment carried out in a vacuum will
   show no difference in the speed of light even if there is an aether.
   According to him, electroweak theory and quantum field theory suggest
   that light could appear to move at different speeds in different
   directions in a medium such as a dense gas in contradiction with
   special relativity; the speed of light would be sensitive to motion
   relative to an ether and the refractive index of the medium. Consoli
   and Evelina Costanzo propose an experiment with laser light passing
   through cavities filled with a relatively dense gas. With the Earth
   passing through an aether wind, light would travel faster in one
   direction than in the perpendicular direction. Consoli and Constanzo
   have not run the proposed experiment. The mathematical treatment of
   their paper does not use the relativistic dragging coefficient to
   account for the speed of light in a moving medium, and most physicists
   regard this as an elementary error that leads to their incorrect
   conclusions. Their paper is very similar to another similarly flawed
   paper by Reg Cahill ("R.T. Cahill A New Light-Speed Anisotropy
   Experiment: Absolute Motion and Gravitational Waves Detected, in
   Progress in Physics, vol 4 , 2006" ), another proponent of an
   experiment that would detect the elusive "preferential frame". Cahill
   claims to have detected absolute motion with respect to a preferential
   frame but his paper suffers from the same mathematical shortcomings as
   the Consoli-Constanzo paper as well as from lack of experimental error
   bars in his experimental data processing. Consequently, their research
   had no impact on the physics community.

Outside the scientific community

   Some adherents of modern geocentrism claim that the Michelson-Morley
   experiment proves that the Earth is stationary which in turn causes
   them to explain the universe in terms of an aether or "firmament". Many
   of these ideas are related to fundamentalist interpretations of
   Christianity.

Aether concepts

     * Aether theories
     * Aether (classical element)
     * Aether drag hypothesis

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