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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,
aether, or
ether, meaning light-bearing
aether, was the postulated
medium for the propagation of
light.
[1] It was invoked to explain the ability of the apparently
wave-based
light to propagate through empty space, something that waves should not
be able to do. The assumption of a spatial plenum of luminiferous
aether, rather than a spatial vacuum, provided the theoretical medium
that was required by wave theories of light.
The concept was the topic of considerable debate throughout its
history, as it required the existence of an invisible and infinite
material with no interaction with physical objects. As the nature of
light was explored, especially in the 19th century, the physical
qualities required of the aether became increasingly contradictory. By
the late 1800s, the existence of the aether was being questioned,
although there was no physical theory to replace it.
The negative outcome of the
Michelson–Morley experiment
(1887) suggested that the aether was non-existent, findings which were
confirmed in subsequent experiments through the 1920s. This led to
considerable theoretical work to explain the propagation of light
without an aether. A major breakthrough was the
theory of relativity,
which could explain why the experiment failed to see aether, but was
more broadly interpreted to suggest that it was not needed. The
Michelson-Morley experiment, along with the
blackbody radiator and
photoelectric effect, was a key experiment in the development of
modern physics, which includes both relativity and
quantum theory, the latter of which explains the wave-like nature of light.
The history of light and aether
Particles vs. waves
To
Robert Boyle
in the 17th century, shortly before Isaac Newton, the aether was a
probable hypothesis and consisted of subtle particles, one sort of which
explained the absence of vacuum and the mechanical interactions between
bodies, and the other sort of which explained phenomena such as
magnetism (and possibly gravity) that were inexplicable on the basis of
the purely mechanical interactions of macroscopic bodies, "though in the
ether of the ancients there was nothing taken notice of but a diffused
and very subtle substance; yet we are at present content to allow that
there is always in the air a swarm of steams moving in a determinate
course between the north pole and the south".
[2]
Isaac Newton
contended that light was made up of numerous small particles. This
could explain such features as light's 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 satisfactory.
[citation needed] To explain refraction, 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", which caused refraction and
diffraction. Newton believed that these vibrations were related to heat
radiation:
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?[A 1]:349
The modern understanding is that heat radiation is, like light,
electromagnetic radiation.
However, Newton viewed heat and light as two different phenomena. He
believed heat vibrations to be excited "when a Ray of Light falls upon
the Surface of any pellucid Body".
[A 1]:348 He wrote, "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 elastic 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.[A 1]:352
Before Newton,
Christiaan Huygens had hypothesized that light was a wave propagating through an aether.
[citation needed] Newton rejected this idea, mainly on the ground that both men apparently could only envision light as a
longitudinal wave, like sound and other
mechanical waves in
fluids.
[citation needed]
However, longitudinal waves necessarily have only one form for a given propagation direction, rather than two
polarizations like
transverse wave.
[citation needed] Thus, longitudinal waves could not explain
birefringence, in which two polarizations of light are refracted differently by a crystal.
[citation needed] Instead, Newton preferred to imagine non-spherical particles, or
"corpuscles", of light with different "sides" that give rise to
birefringence.
[citation needed]
In addition, Newton rejected light as waves in a medium 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".
[citation needed]
Bradley suggests particles
In 1720
James Bradley carried out a series of experiments attempting to measure
stellar parallax
by taking measurements of stars at different times of the year. As the
Earth moves around the sun, the apparent angle to a given distant spot
changes, and by measuring those angles the distance to the star can be
calculated based on the known orbital circumference of the Earth around
the sun. He failed to detect any parallax, thereby placing a lower limit
on the distance to stars.
During these experiments he also discovered a similar effect; the
apparent positions of the stars did change over the year, but not as
expected. Instead of the apparent angle being maximized when the Earth
was at either end of its orbit with respect to the star, the angle was
maximized when the Earth was at its fastest sideways velocity with
respect to the star. This interesting effect is now known as
stellar aberration.
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 aether-based
theory of light was regarded as more problematic. As the aberration
relied on relative velocities, and the measured velocity was dependent
on the motion of the Earth, the aether had to be remaining stationary
with respect to the star as the Earth moved through it. This meant that
the Earth could travel through the aether, a physical medium, with no
apparent effect – precisely the problem that led Newton to reject a wave
model in the first place.
Waves theory triumphs
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 assumed, moreover, that like mechanical waves, light waves required a medium for
propagation, and thus required Huygens's 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.
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.
Electromagnetism
In 1856
Wilhelm Eduard Weber and
Rudolf Kohlrausch
performed an experiment to measure the numerical value of the ratio of
the electromagnetic unit of charge to the electrostatic unit of charge. The result came out to be equal to the product of the speed of light and
the square root of two. The following year,
Gustav Kirchhoff
wrote a paper in which he showed that the speed of a signal along an
electric wire was equal to the speed of light. These are the first
recorded historical links between the speed of light and electromagnetic
phenomena.
James Clerk Maxwell began working on
Faraday's
lines of force. In his 1861 paper
On Physical Lines of Force
he modelled these magnetic lines of force using a sea of molecular
vortices that he considered to be partly made of aether and partly made
of ordinary matter. He derived expressions for the dielectric constant
and the magnetic permeability in terms of the transverse elasticity and
the density of this elastic medium. He then equated the ratio of the
dielectric constant to the magnetic permeability with a suitably adapted
version of Weber and Kohlrausch's result of 1856, and he substituted
this result into Newton's equation for the speed of sound. On obtaining a
value that was close to the speed of light as measured by
Fizeau, Maxwell concluded that light consists in undulations of the same medium that is the cause of electric and magnetic phenomena.
[B 1][B 2][B 3][B 4]
Maxwell had however expressed some uncertainties surrounding the
precise nature of his molecular vortices and so he began to embark on a
purely dynamical approach to the problem. He wrote another famous paper
in 1864 under the title of "
A Dynamical Theory of the Electromagnetic Field" in which the details of the luminiferous medium were less explicit.
[A 2] Although Maxwell did not explicitly mention the sea of molecular vortices, his derivation of
Ampère's circuital law
was carried over from the 1861 paper and he used a dynamical approach
involving rotational motion within the electromagnetic field which he
likened to the action of flywheels. Using this approach to justify the
electromotive force equation (the precursor of the
Lorentz force
equation), he derived a wave equation from a set of eight equations
which appeared in the paper and which included the electromotive force
equation and
Ampère's circuital law.
[A 2]
Maxwell once again used the experimental results of Weber and
Kohlrausch to show that this wave equation represented an
electromagnetic wave that propagates at the speed of light, hence
supporting the view that light is a form of electromagnetic radiation.
The apparent need for a propagation medium for such
Hertzian waves
can be seen by the fact that they consist of perpendicular electric (E)
and magnetic (B or H) waves. The E waves consist of undulating dipolar
electric fields, and all such dipoles appeared to require separated and
opposite electric charges. Electric charge is an inextricable property
of
matter,
so it appeared that some form of matter was required to provide the
alternating current that would seem to have to exist at any point along
the propagation path of the wave. Propagation of waves in a true vacuum
would imply the existence of
electric fields without associated
electric charge, or of electric charge without associated matter. Albeit compatible with Maxwell's equations,
electromagnetic induction
of electric fields could not be demonstrated in vacuum, because all
methods of detecting electric fields required electrically charged
matter.
In addition, 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 along with any
variations that might occur in its supportive medium. Maxwell himself
proposed several mechanical models of aether based on wheels and gears,
and
George Francis FitzGerald even constructed a working model of one of them. These models had to agree with the fact that the electromagnetic waves are
transverse but never
longitudinal.
Problems
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.
[citation needed] Maxwell wrote in
Encyclopædia Britannica:
[A 3]
Aethers were invented for the planets to swim in, to constitute
electric atmospheres and magnetic effluvia, to convey sensations from
one part of our bodies to another, and so on, until all space had been
filled three or four times over with aethers. ... The only aether which
has survived is that which was invented by Huygens to explain the
propagation of light.
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 on behalf of
Lord Rayleigh [3] 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 stated it was actually an invention of the Royal Institution.
[4]
By the early 20th century, aether theory was in trouble. A series of
increasingly complex experiments
had been carried out in the late 19th century to try to detect the
motion of the 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
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, 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.
[B 1][B 2][B 3][B 4]
Relative motion between the Earth and aether
Aether drag
The two most important models, which were aimed to describe the relative motion of the Earth and aether, were
Augustin-Jean Fresnel's (1818) model of the (nearly) stationary aether including a partial aether drag determined by Fresnel's dragging coefficient,
[A 4] and
George Gabriel Stokes' (1844)
[A 5] model of complete aether drag. The latter theory was not considered as correct, since it was not compatible with the
aberration of light, and the auxiliary hypotheses developed to explain this problem were not convincing. Also, subsequent experiments as the
Sagnac effect (1913) also showed that this model is untenable. However, the most important experiment supporting Fresnel's theory 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 travelling through the medium in the same direction as
v from
c/
n to:
[E 1][E 2]
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 became very problematic 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. This implied that there must be a separate
aether for each of the infinitely many frequencies.
Negative aether-drift experiments
The
key difficulty with Fresnel's 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. Sound travels
4.3 times faster in water than in air. This explains why a person
hearing an explosion underwater and quickly surfacing can hear it again
as the slower travelling sound arrives through the air. Similarly, a
traveller on an
airliner
can still carry on a conversation with another traveller because the
sound of words is 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 flying (at least at a constant speed) as
if still sitting on the ground. This is the basis of the Galilean
transformation, and the concept of frame of reference.
But the same was not supposed to be true for light, since Maxwell's
mathematics demanded a single universal speed for the propagation of
light, based, not on local conditions, but on two measured properties,
the
permittivity and
permeability
of free space, 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
colours, for instance.
[verification needed]
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.
First order experiments
Although
the aether is almost stationary according to Fresnel, his theory
predicts a positive outcome of aether drift experiments only to
second order in
,
because Fresnel's dragging coefficient would cause a negative outcome
of all optical experiments capable of measuring effects to
first order in
.
This was confirmed by the following first-order experiments, which all
gave negative results. The following list is based on the description of
Wilhelm Wien (1898), with changes and additional experiments according to the descriptions of
Edmund Taylor Whittaker (1910) and
Jakob Laub (1910):
[B 5][B 1][B 6]
- The experiment of François Arago
(1810), to confirm whether refraction, and thus the aberration of
light, is influenced by Earth's motion. Similar experiments were
conducted by George Biddell Airy (1871) by means of a telescope filled with water, and Éleuthère Mascart (1872).[E 3][E 4][E 5]
- The experiment of Fizeau (1860), to find whether the rotation of the
polarization plane through glass columns is changed by Earth's motion.
He obtained a positive result, but Lorentz could show that the results
have been contradictory. DeWitt Bristol Brace (1905) and Strasser (1907) repeated the experiment with improved accuracy, and obtained negative results.[E 6][E 7][E 8]
- The experiment of Martin Hoek (1868). This experiment is a more precise variation of the famous Fizeau experiment (1851).
Two light rays were sent in opposite directions – one of them traverses
a path filled with resting water, the other one follows a path through
air. In agreement with Fresnel's dragging coefficient, he obtained a
negative result.[E 9]
- The experiment of Wilhelm Klinkerfues
(1870) investigated whether an influence of Earth's motion on the
absorption line of sodium exists. He obtained a positive result, but
this was shown to be an experimental error, because a repetition of the
experiment by Haga (1901) gave a negative result.[E 10][E 11]
- The experiment of Ketteler (1872), in which two rays of an
interferometer were sent in opposite directions through two mutually
inclined tubes filled with water. No change of the interference fringes
occurred. Later, Mascart (1872) showed that the interference fringes of
polarized light in calcite remained uninfluenced as well.[E 12][E 13]
- The experiment of Éleuthère Mascart
(1872) to find a change of rotation of the polarization plane in
quartz. No change of rotation was found when the light rays had the
direction of Earth's motion and then the opposite direction. Lord Rayleigh conducted similar experiments with improved accuracy, and obtained a negative result as well.[E 5][E 13][E 14]
Besides those optical experiments, also electrodynamic first-order
experiments were conducted, which should have led to positive results
according to Fresnel. However,
Hendrik Antoon Lorentz (1895) modified Fresnel's theory and showed that those experiments can be explained by a stationary aether as well:
[A 6]
- The experiment of Wilhelm Röntgen (1888), to find whether a charged condenser produces magnetic forces due to Earth's motion.[E 15]
- The experiment of Theodor des Coudres
(1889), to find whether the inductive effect of two wire rolls upon a
third one is influenced by the direction of Earth's motion. Lorentz
showed that this effect is cancelled to first order by the electrostatic
charge (produced by Earth's motion) upon the conductors.[E 16]
- The experiment of Königsberger (1905). The plates of a condenser are
located in the field of a strong electromagnet. Due to Earth's motion,
the plates should have become charged. No such effect was observed.[E 17]
- The experiment of Frederick Thomas Trouton
(1902). A condenser was brought parallel to Earth's motion, and it was
assumed that momentum is produced when the condenser is charged. The
negative result can be explained by Lorentz's theory, according to which
the electromagnetic momentum compensates the momentum due to Earth's
motion. Lorentz could also show, that the sensitivity of the apparatus
was much too low to observe such an effect.[E 18]
Second order experiments
The Michelson–Morley experiment compared the time for light to reflect from mirrors in two orthogonal directions.
While the
first-order experiments could be explained by a modified stationary aether, more precise
second-order experiments were expected to give positive results, however, no such results could be found.
The famous
Michelson–Morley experiment
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. In this experiment, their goal
was to determine the velocity of the Earth through the aether.
[E 19][E 20] The publication of their result in 1887, the
null result,
was the first clear demonstration that something was seriously wrong
with the aether concept (Michelson's first experiment in 1881 was not
entirely conclusive). In this case the MM experiment yielded a shift of
the fringing pattern of about 0.01 of a
fringe,
corresponding to a small velocity. However, it was incompatible with
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.
Therefore, the
null hypothesis,
the hypothesis that there was no aether wind, could not be rejected.
More modern experiments have since reduced the possible value to a
number very close to zero, about 10
−17.
It is obvious from what has gone before that it would be hopeless to
attempt to solve the question of the motion of the solar system by
observations of optical phenomena at the surface of the earth.
—
A. Michelson and E. Morley. "On the Relative Motion of the Earth and the Luminiferous Æther". Phil. Mag. S. 5. Vol. 24. No. 151. Dec. 1887.[5]
A series of experiments using similar but increasingly sophisticated
apparatuses all returned the null result as well. Conceptually different
experiments that also attempted to detect the motion of the aether were
the
Trouton–Noble experiment (1903),
[E 21] whose objective was to detect
torsion effects caused by electrostatic fields, and
the experiments of Rayleigh and Brace (1902, 1904),
[E 22][E 23] to detect
double refraction in various media. However, all of them obtained a null result, like Michelson–Morley (MM) previously did.
These "aether-wind" experiments led to a flurry of efforts to "save"
aether by assigning to it ever more complex properties, while only few
scientists, like
Emil Cohn or
Alfred Bucherer,
considered the possibility of the abandonment of the aether concept. Of
particular interest was the possibility of "aether entrainment" or
"aether drag", which would lower the magnitude of the measurement,
perhaps enough to explain the results of the Michelson-Morley
experiment. However, as noted earlier, aether dragging already had
problems of its own, notably aberration. In addition, the interference
experiments of
Lodge (1893, 1897) and
Ludwig Zehnder (1895), aimed to show whether the aether is dragged by various, rotating masses, showed no aether drag.
[E 24][E 25][E 26] A more precise measurement was made in the
Hammar experiment (1935), which ran a complete MM experiment with one of the "legs" placed between two massive lead blocks.
[E 27]
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 achieved. 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 by the
Michelson–Gale–Pearson experiment, which detected the Sagnac effect due to Earth's rotation (see
Aether drag hypothesis).
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 the apparatus
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 apparatus itself, cancelling out any
difference when measured. FitzGerald had inferred this hypothesis from a
paper by
Oliver Heaviside. Without referral to an aether, 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.
[E 28][6]
Similarly the
Sagnac effect, observed by G. Sagnac in 1913, was immediately seen to be fully consistent with special relativity.
[E 29][E 30] In fact, the
Michelson-Gale-Pearson experiment
in 1925 was proposed specifically as a test to confirm the relativity
theory, although it was also recognized that such tests, which merely
measure absolute rotation, are also consistent with non-relativistic
theories.
[7]
During the 1920s, the experiments pioneered by Michelson were repeated by
Dayton Miller,
who publicly proclaimed positive results on several occasions, although
they were not large enough to be consistent with any known aether
theory. However, other researchers were unable to duplicate Miller's
claimed results. Over the years the experimental accuracy of such
measurements has been raised by many orders of magnitude, and no trace
of any violations of Lorentz invariance has been seen. (A later
re-analysis of Miller's results concluded that he had underestimated the
variations due to temperature.)
Since the Miller experiment and its unclear results there have been
many more experimental attempts to detect the aether. Many experimenters
have claimed positive results. These results have not gained much
attention from mainstream science, since they contradict a large
quantity of high-precision measurements, all the results of which were
consistent with special relativity.
[8]
Lorentz aether theory
Between 1892 and 1904,
Hendrik Lorentz
developed an electron-aether theory, in which he introduced a strict
separation between matter (electrons) and aether. In his model the
aether is completely motionless, and won't be set in motion in the
neighborhood of ponderable matter. Contrary to earlier electron models,
the electromagnetic field of the aether appears as a mediator between
the electrons, and changes in this field cannot propagate faster than
the speed of light. A fundamental concept of Lorentz's theory in 1895
was the "theorem of corresponding states" for terms of order v/c.
[A 6]
This theorem states that an observer moving relative to the aether
makes the same observations as a resting observer, after a suitable
change of variables. Lorentz noticed that it was necessary to change the
space-time variables when changing frames and introduced concepts like
physical
length contraction (1892)
[A 7] to explain the Michelson–Morley experiment, and the mathematical concept of
local time (1895) to explain the
aberration of light and the
Fizeau experiment. This resulted in the formulation of the so-called
Lorentz transformation by
Joseph Larmor (1897, 1900)
[A 8][A 9] and Lorentz (1899, 1904),
[A 10][A 11] whereby (it was noted by Larmor) the complete formulation of local time is accompanied by some sort of
time dilation
of electrons moving in the aether. As Lorentz later noted (1921, 1928),
he considered the time indicated by clocks resting in the aether as
"true" time, while local time was seen by him as a heuristic working
hypothesis and a mathematical artifice.
[A 12][A 13]
Therefore, Lorentz's theorem is seen by modern authors as being a
mathematical transformation from a "real" system resting in the aether
into a "fictitious" system in motion.
[B 7][B 3][B 8]
The work of Lorentz was mathematically perfected by
Henri Poincaré, who formulated on many occasions the
Principle of Relativity
and tried to harmonize it with electrodynamics. He declared
simultaneity only a convenient convention which depends on the speed of
light, whereby the constancy of the speed of light would be a useful
postulate for making the laws of nature as simple as possible. In 1900 and 1904
[A 14][A 15] he physically interpreted Lorentz's local time as the result of clock synchronization by light signals. In June and July 1905
[A 16][A 17]
he declared the relativity principle a general law of nature, including
gravitation. He corrected some mistakes of Lorentz and proved the
Lorentz covariance of the electromagnetic equations. However, he used
the notion of an aether as a perfectly undetectable medium and
distinguished between apparent and real time, so most historians of
science argue that he failed to invent special relativity.
[B 7][B 9][B 3]
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 theory of relativity, giving the mathematics of
Lorentzian electrodynamics a new, "non-aether" context.
[A 18]
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.
[B 10] 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 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 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".
[A 19]
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 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".
[9][B 7]
Other models
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".
[10][A 20] 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.
[11]
"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."
[12]
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).
[A 21][A 22][A 23][A 24][B 11][B 12][B 13]
In 1918 Einstein publicly alluded to that new definition for the first time.
[A 21]
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
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).
[B 13][A 22][13]
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.
[A 23]
The only similarity of this relativistic aether concept with the
classical aether models lies in the presence of physical properties in
space, which can be identified through
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.
[B 11][B 12][B 13]