Huyghens' wave theory of light, now so generally accepted, loses its
entire significance if a medium for the propagation of these waves
is left out of consideration. This medium we call the ether. [2]
Huyghens' reasoning may be illustrated in some such way as this:
If a body moves a force pushes or pulls it. That force itself is
exemplified in some kind of matter--say a horse. The horse in pulling
a cart is attached to the cart. The horse in pulling a boat may not be
attached to the boat directly but to a rope, which in turn is attached
to the boat. In common cases where one piece of matter affects another,
there is some direct contact, some go-between.
But cases are known where matter affects matter without affording us
any evidence of contact. Take the case of a magnet's attraction for
a piece of iron. Where is the rope that pulls the iron towards the
magnet? Perhaps you think the attraction due to the air in between the
magnet and iron? But removing the air does not stop the attraction. Yet
how can we conceive of the iron being drawn to the magnet unless there
is some go-between? some medium not readily perceptible to the senses
perhaps, and therefore not strictly a form of matter?
If we can but picture some such medium we can imagine our magnet
giving rise to vibrations in this medium which are carried to the
iron. The magnet may give rise to a disturbance in that portion of the
medium nearest to it; then this portion hands over the disturbance
to its neighbor, the next portion of the medium; and so on, until
the disturbance reaches the iron. You see, we are satisfying our
sense-perception by arguing in favor of action by actual contact
rather than some vague action at a distance; the go-between instead
of being a rope is the medium called the ether.
Foucault's experiment completely shattered the corpuscular theory
of light, and for want of any other more plausible alternative, we
are thrown back on Huyghens' wave theory. It will presently appear
that this wave theory has elements in it which make it an excellent
alternative. In the meantime, if light is to be considered as a wave
motion, then the query immediately arises, what is the medium through
which these waves are propagated? If water is the medium for the waves
of the sea, what is the medium for the waves of light? Again we answer,
the medium is the ether.
What Is This "Ether"? Balloonists find conditions more and more
uncomfortable the higher they ascend, for the density of the air
(and therefore the amount of oxygen in a given volume of air) becomes
less and less. Meteorologists have calculated that traces of the
air we breathe may reach a height of some 200 miles. But what is
beyond? Nothing but the ether, it is claimed. Light from the sun and
stars reaches us via the ether.
But what is this ether? We cannot handle it. We cannot see it. It
fails to fall within the scope of any of our senses, for every attempt
to show its presence has failed. It is spirit-like in the popular
sense. It is Lodge's medium for the souls of the departed.
Helmholtz and Kelvin tried to arrive at some properties of this
hypothetical substance from a careful study of the manner in
which waves were propagated through this ether. If, as the wave
theory teaches us, the ether can be set in motion, then according
to laws of mechanics, the ether has mass. If so it is smaller in
amount than anything which can be detected with our most accurate
balance. Further--and this is a difficulty not easily explained--if
this ether has any mass, why does it offer no detectable resistance
to the velocity of the planets in it? Why is not the velocity of
the planets reduced in time, just as the velocity of a rifle bullet
decreases owing to the resistance of the air?
Lodge, in arguing in favor of an ether, holds that its presence cannot
be detected because it pervades all space and all matter. His favorite
analogy is to point out the extreme unlikelihood of a deep-sea fish
discovering the presence of the water with which it is surrounded on
all sides;--all of which tells us nothing about the ether, but does
try to tell us why we cannot detect it. [3]
In short, answering the query at the head of this paragraph, we may
say that we do not know.
Waves Set up in This Ether. The waves are not all of the same
length. Those that produce the sensation of sight are not the smallest
waves known, yet their length is so small that it would take anywhere
from one to two million of them to cover a yard. Curiously enough,
our eye is not sensitive to wave lengths beyond either side of these
limits; yet much smaller, and much larger waves are known. The smallest
are the famous X-rays, which are scarcely one ten-thousandth the size
of light waves. Waves which have a powerful chemical action--those
which act on a photographic plate, for example--are longer than
X-rays, yet smaller than light waves. Waves larger than light waves
are those which produce the sensation of heat, and those used in
wireless telegraphy. The latter may reach the enormous length of 5,000
yards. X-ray, actinic, or chemically active ray, light ray, heat ray,
wireless ray--they differ in size, yet they all have this in common:
they travel with the same speed (186,000 miles per second).
The Electromagnetic Theory of Light. Powerful support to the conception
that space is pervaded by ether was given when Maxwell discovered
light to be an electromagnetic phenomenon. From purely theoretical
considerations this gifted English physicist was led to the view
that waves could be set up as a result of electrical disturbances. He
proved that such waves would travel with the same velocity as light
waves. As air is not needed to transmit electrical phenomena--for you
can pump all air out of a system and produce a vacuum, and electrical
phenomena will continue--Maxwell was forced to the conclusion that
the waves set up by electrical disturbances and transmitted with the
same velocity as light, were enabled to do so with the help of the
same medium as light, namely, the ether.
It was now but a step for Maxwell to formulate the theory that light
itself is nothing but an electrical phenomenon, the sensation of light
being due to the passage of electric waves through the ether. This
theory met with considerable opposition at first. Physicists had been
brought up in a school which had taught that light and electricity
were two entirely unrelated phenomena, and it was difficult for
them to loosen the shackles that bound them to the older school. But
two startling discoveries helped to fasten attention upon Maxwell's
theory. One was an experimental confirmation of Maxwell's theoretical
deduction. Hertz, a pupil of Helmholtz, showed how the discharge from
a Leyden jar set up oscillations, which in turn gave rise to waves in
the ether, comparable, in so far as velocity is concerned, to light
waves, but differing from the latter in wave length, the Hertzian
waves being much longer. At a later date these waves were further
investigated by Marconi, with the result that wireless messages soon
began to be flashed from one place to another.
Just as there is a close connection between light and electricity,
so there is between light and magnetism. The first to point out such
a relationship was the illustrious Michael Faraday, but we owe to
Zeeman the most extensive investigations in this field.
If we throw some common salt into a flame, and, with the help of a
spectroscope, examine the spectrum produced, we are struck by two
bright lines which stand out very prominently. These lines, yellow
in color, are known as the D-lines and serve to identify even minute
traces of sodium. What is true of sodium is true of other elements:
they all produce very characteristic spectra. Now Zeeman found that if
the flame is placed between a powerful magnet, and then some common
salt thrown into the flame, the two yellow lines give place to ten
yellow lines. Such is one of the results of the effect of a magnetic
field on light.
The Electron. The "Zeeman effect" led to several theories regarding
its nature. The most successful of these was one proposed by Larmor and
more fully treated by Lorentz. It has already been pointed out that the
only difference between wireless and light waves is that the former
are much "longer," and, we may now add, their vibrations are much
slower. Light and wireless waves bear a relationship to one another
comparable to the relationship born by high and low-pitched sounds. To
produce wireless waves we allow a charge of electricity to oscillate
to and fro. These oscillations, or oscillating charges, are the cause
of such waves. What charges give rise to light waves? Lorentz, from
a study of the Zeeman effect, ascribed them to minute particles of
matter, smaller than the chemical atom, to which the name "electron"
was given.
The unit of electricity is the electron. Electrons in motion give rise
to electricity, and electrons in vibration, to light. The Zeeman effect
gave Lorentz enough data to calculate the mass of such electrons. He
then showed that these electrons in a magnetic field would be disturbed
by precisely the amount to which Zeeman's observations pointed. In
other words, the assumption of the electron fitted in most admirably
with Zeeman's experiments on magnetism and light.
In the meantime, a study of the discharge of electricity through
gases, and, later, the discovery of radium, led, among other things,
to a study of beta or cathode rays--negatively charged particles
of electricity. Through a series of strikingly original experiments
J. J. Thomson ascertained the mass of such particles or corpuscles, and
then the very striking fact was brought out that Thomson's corpuscle
weighed the same as Lorentz's electron. The electron was not merely
the unit of electricity but the smallest particle of matter.
The Nature of Matter. All matter is made up of some eighty-odd
elements. Oxygen, copper, lead are examples of such elements. Each
element in turn consists of an innumerable number of atoms, of a size
so small, that 300 million of them could be placed alongside of one
another without their total length exceeding one inch.
John Dalton more than a hundred years ago postulated a theory, now
known as the atomic theory, to explain one of the fundamental laws
in chemistry. This theory started out with an old Greek assumption
that matter cannot be divided indefinitely, but that, by continued
subdivision, a point would be reached beyond which no further breaking
up would be possible. The particles at this stage Dalton called atoms.
Dalton's atomic hypothesis became one of the pillars upon which the
whole superstructure of chemistry rested, and this because it explained
a number of perplexing difficulties so much more satisfactorily than
any other hypothesis.
For nearly a century Dalton stood as firm as a rock. But early
in the nineties some epoch-making experiments on the discharge
of electricity through gases were begun by a group of physicists,
particularly Crookes, Rutherford, Lenard, Roentgen, Becquerel, and,
above all, J. J. Thomson, which pointed very clearly to the fact that
the atoms are not the smallest particles of matter at all; that,
in fact, they could be broken up into electrons, of a diameter one
one-hundred-thousandth that of an atom.
It remained for the illustrious Madame Curie to confirm this beyond
all doubt by her isolation of radium. Here, as Madame Curie showed,
was an element whose atoms were actually breaking up under one's very
eyes, so to speak.
So far have we advanced since Dalton's day, that Dalton's unit,
the atom, is now pictured as a complex particle patterned after our
solar system, with a nucleus of positive electricity in the center,
and particles of negative electricity, or electrons, surrounding
the nucleus.
All this leads to one inevitable conclusion: matter is electrical in
nature. But now if matter and light have the same origin, and matter
is subject to gravitation, why not light also? So reasoned Einstein.
Summary. Newton's studies of matter in motion led to his theory of
gravitation, and, incidentally, to his conception of time and space
as definite entities. As we shall see, Einstein in his theory of
gravitation based it upon discoveries belonging to the post-Newtonian
period. One of these is Minkowski's theory of time and space as one
and inseparable. This theory we shall discuss at some length in the
next chapter.
Other important discoveries which led up to Einstein's work are the
researches which culminated in the electron theory of matter. The
origin of this theory may be traced to studies dealing with the nature
of light.
Here again Newton appears as a pioneer. Newton's corpuscular theory,
however, proved wholly untenable when Foucault showed that the
velocity of light in water is less than in air, which is the very
reverse of what the corpuscular theory demands, but which does agree
with Huyghens' wave theory.
But Huyghens' wave theory postulated some medium in which the waves
can act. To this medium the name "ether" was given. However, all
attempts to show the presence of such an ether failed. Naturally
enough, some began to doubt the existence of an ether altogether.
Huyghens' wave theory received a new lease of life with Maxwell's
discovery that light is an electromagnetic phenomenon; that the waves
set up by a source of light were comparable to waves set up by an
electrical disturbance.
Zeeman next showed that magnetism was also, closely related to light.
A study of Zeeman's experiments led Lorentz to the conclusion that
electrical phenomena are due to the motion of charged particles called
"electrons," and that the vibrations of these electrons give rise
to light.
The conception of the electron as the very fundamental of matter was
arrived at in an entirely different way: from studies dealing with
the discharge of electricity through gases and the breaking up of
the atoms of radium.
If matter and light have the same origin, and if matter is subject
to gravitation, why not light also?