James Clerk Maxwell saw his work as a direct extension of that of Michael Faraday, and as such the later reader of Maxwell’s comprehensive treatise on electricity and magnetism might assume Faraday strongly believed in an aether or some sort of medium that would convey all electromagnetic phenomena including light. In his writings, however, Faraday remained cautious about making conclusions that he could not support experimentally, though he sometimes speculated on more aethereal matters. Shortly after his discovery of Faraday rotation, which gave scientists their first clear evidence that light is somehow related to electricity and magnetism, Faraday wrote a letter titled “Thoughts on Ray-Vibrations” to his friend Richard Phillips, which shows that Faraday had a nuanced view of the aether and the nature of light as an electromagnetic phenomenon. For example, he considered an alternative to the aether theory as early as the 1840s. Regarding a lecture he had just given, Faraday wrote the following to his friend Phillips:
The point intended to be set forth for consideration of the hearers was, whether it was not possible that vibrations which in a certain theory are assumed to account for radiation and radiant phenomena may not occur in the lines of force which connect particles, and consequently masses of matter together; a notion which as far as is admitted, will dispense with the aether, which in another view, is supposed to be the medium in which these vibrations take place.
Faraday’s notions were based on a forerunner to the atomic hypothesis put forward by the physicist Roger Boscovich (1711-1787), who speculated that atoms were force centers devoid of matter. Faraday alludes to this in his letter to Phillips:
You are aware of the speculation which I some time since uttered respecting that view of the nature of matter which considers its ultimate atoms as centres of force, and not as so many little bodies surrounded by forces, the bodies being considered in the abstract as independent of the forces and capable of existing without them…The consideration of matter under this view gradually led me to look at the lines of force as being perhaps the seat of vibrations of radiant phenomena.
Faraday compares the speed of light in a vacuum to the speed with which electrical effects travel in a wire, and suggests a connection between matter and radiation:
Another consideration bearing conjointly on the hypothetical view both of matter and radiation, arises from the comparison of the velocities with which the radiant action and certain powers of matter are transmitted. The velocity of light through space is about 190,000 miles in a second; the velocity of electricity is, by the experiments of Wheatstone, shown to be as great as this, if not greater: the light is supposed to be transmitted by vibrations through an aether which is, so to speak, destitute of gravitation, but infinite in elasticity; the electricity is transmitted through a small metallic wire, and is often viewed as transmitted by vibrations also.
That the electric transference depends on the forces or powers of the matter of the wire can hardly be doubted, when we consider the different conductibility of the various metallic and other bodies…The power of electric conduction (being a transmission of force equal in velocity to that of light) appears to be tied up in and dependent upon the properties of the matter, and is, as it were, existent in them.
He then compares the standard theories that build ordinary matter and aether out of material atoms, or “nuclei,” with his and Boscovich’s conception of such atoms as mere “centers of force” that are “devoid of matter”:
I suppose we may compare together the matter of the aether and ordinary matter (as, for instance, the copper of the wire through which the electricity is conducted), and consider them as alike in their essential constitution; i.e. either as both composed of little nuclei, considered in the abstract as matter, and of force or power associated with these nuclei, or else both consisting of mere centres of force, according to Boscovich’s theory and the view put forth in my speculation; for there is no reason to assume that the nuclei are more requisite in the one case than in the other. It is true that the copper gravitates and the aether does not, and that therefore the copper is ponderable and the aether is not; but that cannot indicate the presence of nuclei in the copper more than in the aether, for of all the powers of matter gravitation is the one in which the force extends to the greatest possible distance from the supposed nucleus, being infinite in relation to the size of the latter, and reducing the nucleus to a mere centre of force. The smallest atom of matter on the earth acts directly on the smallest atom of matter in the sun, though they are 95,000,000 miles apart; further, atoms which, to our knowledge, are at least nineteen times that distance, and indeed in cometary masses, far more, are in a similar way tied together by the lines of force extending from and belonging to each. What is there in the condition of the particles of the supposed aether, if there be even only one such particle between us and the sun, that can in subtlety and extent compare to this?
In experimental philosophy we can, by the phenomena presented, recognize various kinds of lines of force; thus there are the lines of gravitating force, those of electro-static induction, those of magnetic action, and others partaking of a dynamic character might be perhaps included. The lines of electric and magnetic action are by many considered as exerted through space like the lines of gravitating force. For my own part, I incline to believe that when there are intervening particles of matter (being themselves only centres of force), they take part in carrying on the force through the line, but that when there are none, the line proceeds through space. Whatever the view adopted respecting them may be, we can, at all events, affect these lines of force in a manner which may be conceived as partaking of the nature of a shake or lateral vibration. For suppose two bodies, A B, distant from each other and under mutual action, and therefore connected by lines of force, and let us fix our attention upon one resultant of force, having an invariable direction as regards space; if one of the bodies move in the least degree right or left, or if its power be shifted for a moment within the mass (neither of these cases being difficult to realise if A and B be either electric or magnetic bodies), then an effect equivalent to a lateral disturbance will take place in the resultant upon which we are fixing our attention; for, either it will increase in force whilst the neighboring results are diminishing, or it will fall in force as they are increasing.
It may be asked, what lines of force are there in nature which are fitted to convey such an action and supply for the vibrating theory the place of the aether? I do not pretend to answer this question with any confidence; all I can say is, that I do not perceive in any part of space, whether (to use the common phrase) vacant or filled with matter, anything but forces and the lines in which they are exerted. The lines of weight or gravitating force are, certainly, extensive enough to answer in this respect any demand made upon them by radiant phenomena; and so, probably, are the lines of magnetic force: and then who can forget that Mossotti has shown that gravitation, aggregation, electric force, and electro-chemical action may all have one common connection or origin; and so, in their actions at a distance, may have in common that infinite scope which some of these actions are known to possess? [1]
The view which I am so bold to put forth considers, therefore, radiation as a kind of species of vibration in the lines of force which are known to connect particles and also masses of matter together. It endeavors to dismiss the aether, but not the vibration. The kind of vibration which, I believe, can alone account for the wonderful, varied, and beautiful phenomena of polarization, is not the same as that which occurs on the surface of disturbed water, or the waves of sound in gases or liquids, for the vibrations in these cases are direct, or to and from the centre of action, whereas the former are lateral. It seems to me, that the resultant of two or more lines of force is in an apt condition for that action which may be considered as equivalent to a lateral vibration; whereas a uniform medium, like the aether, does not appear apt, or more apt than air or water.
The occurrence of a change at one end of a line of force easily suggests a consequent change at the other. The propagation of light, and therefore probably of all radiant action, occupies time; and, that a vibration of the line of force should account for the phenomena of radiation, it is necessary that such vibration should occupy time also. I am not aware whether there are any data by which it has been, or could be ascertained whether such a power as gravitation acts without occupying time, or whether lines of force being already in existence, such a lateral disturbance at one end as I have suggested above, would require time, or must of necessity be felt instantly at the other end.
Faraday explains how his conception of lines of force would obviate the need for a the hypothesized aether:
The aether is assumed as pervading all bodies as well as space: in the view now set forth, it is the forces of the atomic centres which pervade (and make) all bodies, and also penetrate all space. As regards space, the difference is, that the aether presents successive parts of centres of action, and the present supposition only lines of action; as regards matter, the difference is, that the aether lies between the particles and so carries on the vibrations, whilst as respects the supposition, it is by the lines of force between the centres of the particles that the vibration is continued. As to the difference in intensity of action within matter under the two views, I suppose it will be very difficult to draw any conclusion, for when we take the simplest state of common matter and that which most nearly causes it to approximate to the condition of the aether, namely the state of the rare gas, how soon do we find in its elasticity and the mutual repulsion of its particles, a departure from the law, that the action is inversely as the square of the distance! [2]
[1] Faraday is referring to a theory advanced by the Italian physicist Ottaviano-Fabrizio Mossotti (1791–1863), which purportedly derived the gravitational and electrical force laws from a more general law. The theory ultimately proved unsuccessful, but Faraday considered it for a while. For a further discussion of Mossotti’s theory, see James F. Woodward, “Early attempts at a Unitary Understanding of Nature,” published in Old and New Questions in Physics, Cosmology, Philosophy, and Theoretical Biology, ed. Awyn van der Merwe, (New York: Plenum Press, 1983), 886-894 [885-908].
[2] M. Faraday, “Experimental Researches in Electricity,” Vol. III , 447-452, and M. Faraday, Philosophical Magazine, S.3, Vol XXVIII, N188, May 1846.
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