The Luminiferous Ether

The properties of matter fail to explain some physical phenomena.

Descartes had the first idea of attributing such phenomena to some subtle matter which is the receptacle of the energy of the universe.

In our times this idea has had extraordinary luck.

• It had been eclipsed for 200 years by the success of the immortal synthesis of Newton.
• But it gained an entirely new splendour with Fresnel and his followers.

Thanks to their admirable discoveries, the first stage seemed accomplished.

• The laws of optics were represented by a single hypothesis, marvellously fitted to allow us to anticipate unknown phenomena.

All these anticipations were subsequently fully verified by experiment.

• But the researches of Faraday, Maxwell, and Hertz authorized still greater ambitions.

This medium was called by the ancients as “ether”.

• It had already explained light and radiant heat.
• It would also be sufficient to explain electricity.

Thus, the hope began to take form that we might succeed in demonstrating the unity of all physical forces.

It was thought that the knowledge of the laws relating to the inmost movements of this ether might:

• give us the key to all phenomena, and
• make us acquainted with the method in which energy is stored up, transmitted, and parcelled out in its external manifestations.

We cannot study here all the problems which are connected with the physics of the ether. To do this a complete treatise on optics would have to be written and a very lengthy one on electricity.

I shall simply show rapidly how in the last few years the ideas relative to the constitution of this ether have evolved.

Is it possible without self-delusion to imagine that a single medium can really allow us to group all the known facts in one comprehensive arrangement?

Fresnel’s hypothesis was that of the luminous ether.

• This hypothesis:
  • had so great a struggle at the outset to overcome the stubborn resistance of the partisans of the then classic theory of emission.
  • seemed to have in the sequel an unshakable strength

Lamé was a prudent mathematician. He wrote:

“The existence of the ethereal fluid is incontestably demonstrated by the propagation of light through the planetary spaces, and by the explanation, so simple and so complete, of the phenomena of diffraction in the wave theory of light.. The laws of double refraction prove with no less certainty that the ether exists in all diaphanous media.”

Thus, the ether was no longer an hypothesis, but in some sort a tangible reality.

But the ethereal fluid of which the existence was thus proclaimed has some singular properties.

Laplace and Poisson made objections against the ether.

• These are treated somewhat lightly at the present day.
• But these have not lost all value

It was supposed to explain rectilinear propagation, reflexion, refraction, diffraction, and interferences despite grave difficulties at the outset.

• The only hypothesis that came from it was that the undulations of an elastic medium called the ether caused all those movements.
• It did not explain the nature and direction of those vibrations.

This ether medium was said to exist in the void.

• Therefore, it is considered as imponderable.

It may be compared to a fluid of negligible mass.

• It offers no appreciable resistance to the motion of the planets.
• Instead, it is endowed with an enormous elasticity, because the speed of light is considerable.

It must be capable of penetrating into all transparent bodies, and of retaining there as a constant elasticity.

• But there, it must become condensed, since the speed of propagation in these bodies is less than in a vacuum.

Such properties belong to no material gas, even the most rarefied.

The important point is that they admit of no essential contradiction. [20]

It was the study of the phenomena of polarization which led Fresnel to his bold conception of transverse vibrations, and subsequently induced him to penetrate further into the constitution of the ether.

We know the experiment of Arago on the noninterference of polarized rays in rectangular planes.

While 2 systems of waves, proceeding from the same source of natural light and propagating themselves in nearly parallel directions, increase or become destroyed according to whether the nature of the superposed waves are of the same or of contrary signs, the waves of the rays polarized in perpendicular planes, on the other hand, can never interfere with each other.

Whatever the difference of their course, the intensity of the light is always the sum of the intensity of the two rays.

Fresnel perceived that this experiment absolutely compels us to reject the hypothesis of longitudinal vibrations acting along the line of propagation in the direction of the rays.

To explain it, it must of necessity be admitted, on the contrary, that the vibrations are transverse and perpendicular to the ray.

Verdet could say, in all truth, “It is not possible to deny the transverse direction of luminous vibrations, without at the same time denying that light consists of an undulatory movement.”

Such vibrations do not and cannot exist in any medium resembling a fluid.

The characteristic of a fluid is that its different parts can displace themselves with regard to one another without any reaction appearing so long as a variation of volume is not produced. There certainly may exist, as we have seen, certain traces of rigidity in a liquid, but we cannot conceive such a thing in a body infinitely more subtle than rarefied gas.

Among material bodies, a solid alone really possesses the rigidity sufficient for the production within it of transverse vibrations and for their maintenance during their propagation.

Since we have to attribute such a property to the ether, we may add that on this point it resembles a solid.

Lord Kelvin has shown that this solid, would be much more rigid than steel. This conclusion produces great surprise in all who hear it for the first time, and it is not rare to hear it appealed to as an argument against the actual existence of the ether.

It does not seem that such an argument can be decisive. There is no reason for supposing that the ether ought to be a sort of extension of the bodies we are accustomed to handle.

Its properties may astonish our ordinary way of thinking, but this rather unscientific astonishment is not a reason for doubting its existence.

Real difficulties would appear only if we were led to attribute to the ether, not singular properties which are seldom found united in the same substance, but properties logically contradictory. In short, however odd such a medium may appear to us, it cannot be said that there is any absolute incompatibility between its attributes.

We could even suggest images capable of representing these contrary appearances.

• Various authors have done so.

M. Boussinesq assumes that the ether behaves like a very rarefied gas in respect of the celestial bodies, because these last move, while bathed in it, in all directions and relatively slowly, while they permit it to retain, so to speak, its perfect homogeneity.

On the other hand, its own undulations are so rapid that so far as they are concerned the conditions become very different, and its fluidity has, one might say, no longer the time to come in. Hence its rigidity alone appears.

Another consequence, very important in principle, of the fact that vibrations of light are transverse, has been well put in evidence by Fresnel.

He showed how we have, in order to understand the action which excites without condensation the sliding of successive layers of the ether during the propagation of a vibration, to consider the vibrating medium as being composed of molecules separated by finite distances.

Certain authors have proposed theories in which the action at a distance of these molecules are replaced by actions of contact between parallelepipeds sliding over one another.

But, at bottom, these two points of view both lead us to conceive the ether as a discontinuous medium, like matter itself. The ideas gathered from the most recent experiments also bring us to the same conclusion.