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Theories of the Aether Articles relating to the Emergence of
Scientific Theories of the Cosmic Aether
"Thus he then classified living creatures into genera and species,
and divided them in every way until he came to their elements,
which he called the five shapes and bodies - aether, fire, water, earth and air."
- Xenocrates, On the life of Plato
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Aether and the Theory of Relativity
- Albert Einstein (1920)
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Ether and the Theory of Relativity |
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Albert Einstein
An address delivered on May 5th, 1920, in the University of Leyden
HOW does it come about that
alongside of the idea of ponderable
matter, which is derived by abstraction from everyday life, the physicists
set the idea of the existence of another
kind of matter, the ether? The explanation is probably to be sought in those
phenomena which have given rise to the
theory of action at a distance, and in the
properties of light which have led to the
undulatory theory. Let us devote a little
while to the consideration of these two
subjects.
Outside of physics we know nothing
of action at a distance. When we try
to connect cause and effect in the experiences which natural objects afford us,
it seems at first as if there were no other
mutual actions than those of immediate
contact, e.g. the communication of motion
by impact, push and pull, heating or inducing combustion by means of a flame, etc.
It is true that even in everyday experience weight, which is in a sense action at
a distance, plays a very important part.
But since in daily experience the weight
of bodies meets us as something constant,
something not linked to any cause which
is variable in time or place, we do not
in everyday life speculate as to the cause
of gravity, and therefore do not become
conscious of its character as action at
a distance. It was Newton's theory of
gravitation that first assigned a cause
for gravity by interpreting it as action
at a distance, proceeding from masses.
Newton's theory is probably the greatest
stride ever made in the effort towards
the causal nexus of natural phenomena.
And yet this theory evoked a lively sense
of discomfort among Newton's contemporaries, because it seemed to be in conflict
with the principle springing from the rest
of experience, that there can be reciprocal
action only through contact, and not
through immediate action at a distance.
It is only with reluctance that man's
desire for knowledge endures a dualism
of thls kind. How was unity to be preserved in his comprehension of the forces
of nature? Either by trying to look
upon contact forces as being themselves
distant forces which admittedly are observable only at a very small distance
and this was the road which Newton's
followers, who were entirely under the
spell of his doctrine, mostly preferred
to take; or by assuming that the Newtonian action at a distance is only apparently
immediate action at a distance, but in
truth is conveyed by a medium permeating space, whether by movements or by
elastic deformation of this medium. Thus
the endeavour toward a unified view of
the nature of forces leads to the hypothesis
of an ether. This hypothesis, to be sure,
did not at first bring with it any advance
in the theory of gravitation or in physics
generally, so that it became customary
to treat Newton's law of force as an axiom
not further reducible. But the ether hypothesis was bound always to play some
part in physical science, even if at first
only a latent part.
When in the first half of the nineteenth
century the far-reaching similarity was revealed which subsists between the properties
of light and those of elastic waves in ponderable bodies, the ether hypothesis found
fresh support. 1t appeared beyond question
that light must be interpreted as a vibratory
process in an elastic, inert medium filling
up universal space. It also seemed to
be a necessary consequence of the fact
that light is capable of polarisation that
this medium, the ether, must be of the
nature of a solid body, because transverse
waves are not possible in a fluid, but
only in a solid. Thus the physicists were
bound to arrive at the theory of the ``quas-irigid'' luminiferous ether, the parts of
which can carry out no movements
relatively to one another except the small
movements of deformation which correspond to light-waves.
This theory also called the theory of
the stationary luminiferous ether moreover found a strong support in an experiment which is also of fundamental importance in the special theory of relativity,
the experiment of Fizeau, from which
one was obliged to infer that the luminiferous ether does not take part in the
movements of bodies. The phenomenon
of aberration also favoured the theory
of the quasi-rigid ether.
The development of the theory of electricity along the path opened up by Maxwell
and Lorentz gave the development of our
ideas concerning the ether quite a peculiar
and unexpected turn. For Maxwell himself the ether indeed still had properties
which were purely mechanical, although
of a much more complicated kind than
the mechanical properties of tangible solid
bodies. But neither Maxwell nor his
followers succeeded in elaborating a
mechanical model for the ether which
might furnish a satisfactory mechanical
interpretation of Maxwell's laws of the
electro-magnetic field. The laws were clear
and simple, the mechanical interpretations
clumsy and contradictory. Almost imperceptibly the theoretical physicists adapted
themselves to a situation which, from the
standpoint of their mechanical programme,
was very depressing. They were particularly influenced by the electro-dynamical
investigations of Heinrich Hertz. For
whereas they previously had required of
a conclusive theory that it should content itself with the fundamental concepts
which belong exclusively to mechanics
(e.g. densities, velocities, deformations,
stresses) they gradually accustomed themselves to admitting electric and magnetic
force as fundamental concepts side by
side with those of mechanics, without
requiring a mechanical interpretation for
them. Thus the purely mechanical view
of nature was gradually abandoned. But
this change led to a fundamental dualism
which in the long-run was insupportable.
A way of escape was now sought in the
reverse direction, by reducing the principles
of mechanics to those of electricity, and
this especially as confidence in the strict
validity of the equations of Newton's
mechanics was shaken by the experiments with b-rays and rapid kathode
rays.
This dualism still confronts us in unextenuated form in the theory of Hertz,
where matter appears not only as the
bearer of velocities, kinetic energy, and
mechanical pressures, but also as the
bearer of electromagnetic fields. Since
such fields also occur in vacuo i.e. in free
ether the ether also appears as bearer of
electromagnetic fields. The ether appears
indistinguishable in its functions from
ordinary matter. Within matter it takes
part in the motion of matter and in empty
space it has everywhere a velocity; so
that the ether has a definitely assigned
velocity throughout the whole of space.
There is no fundamental difference between
Hertz's ether and ponderable matter (which
in part subsists in the ether).
The Hertz theory suffered not only
from the defect of ascribing to matter and
ether, on the one hand mechanical states,
and on the other hand electrical states,
which do not stand in any conceivable
relation to each other; it was also at variance with the result of Fizeau's important
experiment on the velocity of the propagation of light in moving fluids, and with
other established experimental results.
Such was the state of things when H. A.
Lorentz entered upon the scene. He brought
theory into harmony with experience by
means of a wonderful simplification of
theoretical principles. He achieved this,
the most important advance in the theory
of electricity since Maxwell, by taking
from ether its mechanical, and from matter
its electromagnetic qualities. As in empty
space, so too in the interior of material
bodies, the ether, and not matter viewed
atomistically, was exclusively the seat
of electromagnetic fields. According to
Lorentz the elementary particles of matter
alone are capable of carrying out movements; their electromagnetic activity is
entirely confined to the carrying of electric
charges. Thus Lorentz succeeded in reducing all electromagnetic happenings to
Maxwell's equations for free space.
As to the mechanical nature of the
Lorentzian ether, it may be said of it,
in a somewhat playful spirit, that immobility is the only mechanical property
of which it has not been deprived by
H. A. Lorentz. 1t may be added that the
whole change in the conception of the
ether which the special theory of relativity
brought about, consisted in taking away
from the ether its last mechanical quality,
namely, its immobility. How this is to
be understood will forthwith be expounded.
The space-time theory and the kinematics of the special theory of relativity
were modelled on the Maxwell-Lorentz
theory of the electromagnetic field. This
theory therefore satisfies the conditions
of the special theory of relativity, but
when viewed from the latter it acquires
a novel aspect. For if K be a system
of co-ordinates relatively to which the
Lorentzian ether is at rest, the Maxwell-Lorentz equations are valid primarily with
reference to K. But by the special theory
of relativity the same equations without any change of meaning also hold in
relation to any new system of co-ordinates
K' which is moving in uniform translation relatively to K. Now comes the
anxious question: Why must I in the
theory distinguish the K system above all
K' systems, which are physically equivalent
to it in all respects, by assuming that the
ether is at rest relatively to the K system?
For the theoretician such an asymmetry
in the theoretical structure, with no corresponding asymmetry in the system of
experience, is intolerable. If we assume
the ether to be at rest relatively to K,
but in motion relatively to K', the physical
equivalence of K and K' seems to me
from the logical standpoint, not indeed
downright incorrect, but nevertheless
inacceptable.
The next position which it was possible
to take up in face of this state of things
appeared to be the following. The ether
does not exist at all. The electromagnetic fields are not states of a medium,
and are not bound down to any bearer,
but they are independent realities which
are not reducible to anything else, exactly
like the atoms of ponderable matter. This
conception suggests itself the more readily
as, according to Lorentz's theory, electromagnetic radiation, like ponderable matter,
brings impulse and energy with it, and
as, according to the special theory of
relativity, both matter and radiation are
but special forms of distributed energy,
ponderable mass losing its isolation and
appearing as a special form of energy.
More careful reflection teaches us, however, that the special theory of relativity
does not compel us to deny ether. We
may assume the existence of an ether,;
only we must give up ascribing a definite
state of motion to it, i.e. we must by
abstraction take from it the last mechanical characteristic which Lorentz had still
left it. We shall see later that this point
of view, the conceivability of which
shall at once endeavour to make more
intelligible by a somewhat halting comparison, is justified by the results of
the general theory of relativity.
Think of waves on the surface of water.
Here we can describe two entirely different things. Either we may observe how
the undulatory surface forming the boundary between water and air alters in the
course of time; or else with the help
of small floats, for instance we can
observe how the position of the separate
particles of water alters in the course
of time. If the existence of such floats
for tracking the motion of the particles
of a fluid were a fundamental impossibility in physics if, in fact, nothing
else whatever were observable than the
shape of the space occupied by the water
as it varies in time, we should have no
ground for the assumption that water
consists of inovable particles. But all
the same we could characterise it as a
medium.
We have something like this in the
electromagnetic field. For we may picture the field to ourselves as consisting
of lines of force. If we wish to interpret these lines of force to ourselves as
something inaterial in the ordinary sense,
we are tempted to interpret the dynamic
processes as motions of these lines of
force, such that each separate line of
force is tracked through the course of
time. It is well known, however, that
this way of regarding the electromagnetic
field leads to contradictions.
Generalising we must say this: There
inay be supposed to be extended physical
objects to which the idea of motion cannot be applied. They may not be thought
of as consisting of particles which allow
themselves to be separately tracked through
time. In Minkowski's idiom this is expressed as follows: Not every extended
conformation in the four-dimensional world
can be regarded as composed of worldthreads. The special theory of relativity
forbids us to assume the ether to consist
of particles observable through time, but
the hypothesis of ether in itself
in conflict with the special theory of
relativity. Only we must be on our guard
against ascribing a state of motion to the
ether.
Certainly, from the standpoint of the
special theory of relativity, the ether
hypothesis appears at first to be an empty
hypothesis. 1n the equations of the
electromagnetic field there occur, in addition
to the densities of the electric charge,
only the intensities of the field. The
career of electromagnetic processes in vacuo
appears to be completely determined by
tliese equations, uninfluenced by other physical quantities. The electromagnetic fields
appear as ultimate, irreducible realities,
and at first it seems superfluous to postulate a homogeneous, isotropic ether-medium, and to envisage electromagnetic
fields as states of this medium.
But on the other hand there is a weighty
argument to be adduced in favour of the
ether hypothesis. To deny the ether is
nltimately to assuine that empty space
has no physical qualities whatever. The
fundamental facts of mechanics do not
harmonize with this view. For the mechanical behaviour of a corporeal system
hovering freely in empty space depends
not only on relative positions (distances)
and relative velocities, but also on its
state of rotation, which physically may
be taken as a characteristic not appertaining to the system in itself. In order
to be able to look upon the rotation of
the system, at least formally, as something real, Newton objectivises space.
Since he classes his absolute space together
with real things, for him rotation relative
to an absolute space is also something
real. Newton might no less well have
called his absolute space ``Ether''; what
is essential is merely that besides observable objects, another thing, which is
not perceptible, inust be looked upon as
real, to enable acceleration or rotation
to be looked upon as something real.
It is true that Mach tried to avoid
having to accept as real something which
is not observable by endeavouring to
substitute in inechanics a mean acceleration with reference to the totality of the
masses in the universe in place of an
acceleration with reference to absolute
space. But inertial resistance opposed to
relative acceleration of distant masses
presupposes action at a distance; and
as the modern physicist does not believe
that he may accept this action at a distance, he comes back once inore, if he
follows Mach, to the ether, which has
to serve as medium for the effects of
inertia. But this conception of the ether
to which we are led by Mach's way of
thinking differs essentially from the ether
as conceived by Newton, by Fresnel, and
by Lorentz. Mach's ether not only conditions the behaviour of inert masses,
but is also conditioned in its state by
them.
Mach's idea finds its full development in
the ether of the general theory of relativity.
According to this theory the metrical
qualities of the continuum of space-time
differ in the environment of different points
of space-time, and are partly conditioned
by the matter existing outside of the
territory under consideration. This space-time variability of the reciprocal relations
of the standards of space and time, or,
perhaps, the recognition of the fact that
``empty space'' in its physical relation is
neither homogeneous nor isotropic, compelling us to describe its state by ten functions
(the gravitation potentials g), has, I
think, finally disposed of the view that
space is physically empty. But therewith
the conception of the ether has again
acquired an intelligible content, although
this content differs widely from that of the
ether of the mechanical undulatory theory
of light. The ether of the general theory
of relativity is a medium which is itself
devoid of all mechanical and kinematical
qualities, but helps to determine mechanical
(and electromagnetic) events.
What is fundamentally new in the ether
of the general theory of relativity as opposed to the ether of Lorentz consists in
this, that the state of the former is at
every place determined by connections with
the matter and the state of the ether in
neighbouring places, which are amenable
to law in the form of differential equations,;
whereas the state of the Lorentzian ether
in the absence of electromagnetic fields is
conditioned by nothing outside itself, and
is everywhere the same. The ether of
the general theory of relativity is transmuted conceptually into the ether of
Lorentz if we substitute constants for the
functions of space which describe the
former, disregarding the causes which condition its state. Thus we may also say,
I think, that the ether of the general theory
of relativity is the outcome of the Lorentzian ether, through relativation.
As to the part which the new ether is
to play in the physics of the future we are
not yet clear. We know that it determines
the metrical relations in the space-time
continuum, e.g. the configurative possibilities of solid bodies as well as the gravitational fields; but we do not know whether
it has an essential share in the structure of
the electrical elementary particles constituting matter. Nor do we know whether
it is only in the proximity of ponderable
masses that its structure differs essentially
from that of the Lorentzian ether; whether
the geometry of spaces of cosmic extent
is approximately Euclidean. But we can
assert by reason of the relativistic equations
of gravitation that there must be a departure from Euclidean relations, with spaces
of cosmic order of magnitude, if there
exists a positive mean density, no matter
how small, of the matter in the universe.
In this case the universe must of necessity
be spatially unbounded and of finite magnitude, its inagnitude being determined by
the value of that inean density.
If we consider the gravitational field and
the electromagnetic field from the standpoint of the ether hypothesis, we find a remarkable difference between the two. There
can be no space nor any part of space without
gravitational potentials; for these confer
upon space its metrical qualities, without
which it cannot be imagined at all. The
existence of the gravitational field is
inseparably bound up with the existence
of space. On the other hand a part of
space may very well be imagined without
an electromagnetic field; thus in contrast
with the gravitational field, the electromagnetic field seems to be only secondarily
linked to the ether, the formal nature of
the electromagnetic field being as yet in
no way determined by that of gravitational
ether. From the present state of theory
it looks as if the electromagnetic field, as
opposed to the gravitational field, rests upon
an entirely new formal motif, as though
nature might just as well have endowed the
gravitational ether with fields of quite
another type, for example, with fields of a
scalar potential, instead of fields of the
electromagnetic type.
Since according to our present conceptions the elementary particles of matter
are also, in their essence, nothing else than
condensations of the electromagnctic field,
our present view of the universe presents
two realities which are completely separated
from each other conceptually, although
connected causally, namely, gravitational
ether and electromagnetic field, or as
they might also be called space and
matter.
Of course it would be a great advance if
we could succeed in comprehending the
gravitational field and the electromagnetic
field together as one unified conformation.
Then for the first time the epoch of theoretical physics founded by Faraday and
Maxwell would reach a satisfactory conclusion. The contrast between ether and
matter would fade away, and, through the
general theory of relativity, the whole of
physics would become a complete system of
thought, like geometry, kinematics, and the
theory of gravitation. An exceedingly
ingenious attempt in this direction has
been made by the mathematician H. Weyl,;
but I do not believe that his theory
will hold its ground in relation to reality.
Further, in contemplating the immediate
future of theoretical physics we ought not
unconditionally to reject the possibility
that the facts comprised in the quantum
theory may set bounds to the field theory
beyond which it cannot pass.
Recapitulating, we may say that according to the general theory of relativity space
is endowed with physical qualities; in this
sense, therefore, there exists an ether.
According to the general theory of relativity
space without ether is unthinkable; for
in such space there not only wonld 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 ether may not be thought
of as endowed with the quality characteristic of ponderable inedia, as consisting of
parts which may be tracked through time.
The idea of motion may not be applied
to it.
[Index Document - Theories
of Aether][Quotes of Albert Einstein
]
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Theories of the Aether Articles relating to the Emergence of
Scientific Theories of the Cosmic Aether
"Thus he then classified living creatures into genera and species,
and divided them in every way until he came to their elements,
which he called the five shapes and bodies - aether, fire, water, earth and air."
- Xenocrates, On the life of Plato
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Aether and the Theory of Relativity
- Albert Einstein (1920)
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