Maxwell J.C. “Ether” // Britannica, 9 ed., vol. 8, 1878

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{569} The hypothesis of an aether has been maintained by different speculators for very different reasons. To those who maintained the existence of a plenum as a philosophical principle, nature's abhorrence of a vacuum was a sufficient reason for imagining an all-surrounding aether, even though every other argument should be against it. To Descartes, who made extension the sole essential property of matter, and matter a necessary condition of extension, the bare existence of bodies apparently at a distance was a proof of the existence of a continuous medium between them. But besides these high metaphysical necessities for a medium, there were more mundane uses to be fulfilled by aethers. 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, till all space had been filled three or four times over with aethers. It is only when we remember the extensive and mischievous influence on science which hypotheses about aethers used formerly to exercise, that we can appreciate the horror of aethers which sober-minded men had during the 18th century, and which, probably as a sort of hereditary prejudice, descended even to the late Mr John Stuart Mill. The disciples of Newton maintained that in the fact of the mutual gravitation of the heavenly bodies, according to Newton's law, they had a complete quantitative account of their motions; and they endeavoured to follow out the path which Newton had opened up by investigating and measuring the attractions and repulsions of electrified and magnetic bodies, and the cohesive forces in the interior of bodies, without attempting to account for these forces. Newton himself, however, endeavoured to account for gravitation by differences of pressure in an aether (see Art. Attraction, Vol. iii. p. 64); but he did not publish his theory, “because he was not able from experiment and observation to give a satisfactory account of this medium, and the manner of its operation in producing the chief phenomena of nature.” On the other hand, those who imagined aethers in order to explain phenomena could not specify the nature of the motion of these media, and could not prove that the media, as imagined by them, would produce the effects they were meant to explain. The only aether which has survived is that which was invented by Huygens to explain the propagation of light. The evidence for the existence of the luminiferous aether has accumulated as additional phenomena of light and other radiations have been discovered; and the properties of this medium, as deduced from the phenomena of light, have been found to be precisely those required to explain electromagnetic phenomena. Function of the aether in the propagation of radiation. The evidence for the undulatory theory of light will be given in full, under the Article on Light, but we may here give a brief summary of it so far as it bears on the existence of the aether. That light is not itself a substance may be proved from the phenomenon of interference. A beam of light from a single source is divided by certain optical methods into two parts, and these, after travelling by different paths, are made to reunite and fall upon a screen. If either half of the beam is stopped, the other falls on the screen and illuminates it, but if both are allowed to pass, the screen in certain places becomes dark, and thus shows that the two portions of light have destroyed each other. Now, we cannot suppose that two bodies when put together can annihilate each other; therefore light cannot be a substance. What we have proved is that one portion of light can be the exact opposite of another portion, just as +a is the exact opposite of –a, whatever a may be. Among physical quantities we find some which are capable of having their signs reversed, and others which are not. Thus a displacement in one direction is the exact opposite of an equal displacement in the opposite direction. Such quantities are the measures, not of substances, but always of processes taking place in a substance. We therefore conclude that light is not a substance but a process going on in a substance, the process going on in the first portion of light being always the exact opposite of the process going on in the other at the same instant, so that when the two portions are combined no process goes on at all. To determine the nature of the process in which the radiation of light consists, we alter the length of the path of one or both of the two portions of the beam, and we find that the light is extinguished when the difference of the length of the paths is an odd multiple of a certain small distance called a half wave-length. In all other cases there is more or less light ; and when the paths are equal, or when their difference is a multiple of a whole wave-length, the screen appears four times as bright as when one portion of the beam falls on it. In the ordinary form of the experiment these different cases are exhibited simultaneously at different points of the screen, so that we see on the screen a set of fringes consisting of dark lines at equal intervals, with bright bands of graduated intensity between them. If we consider what is going on at different points in the axis of a beam of light at the same instant, we shall find that if the distance between the points is a multiple of a wave-length the same process is going on at the two points at the same instant, but if the distance is an odd multiple of half a wave-length the process going on at one point is the exact opposite of the process going on at the other. Now, light is known to be propagated with a certain velocity (3.004 × 1010 centimetres per second in vacuum, according to Cornu). If, therefore, we suppose a movable point to travel along the ray with this velocity, we shall find the same process going on at every point of the ray as the moving point reaches it. If, lastly, we consider a fixed point in the axis of the beam, we shall observe a rapid alternation of these opposite processes, the interval of time between similar processes being the time light takes to travel a wave-length. These phenomena may be summed up in the mathematical expression u = A cos(nt – px + a) which gives u, the phase of the process, at a point whose distance measured from a fixed point in the beam is x, and at a time t. We have determined nothing as to the nature of the process. It may be a displacement, or a rotation, or an electrical disturbance, or indeed any physical quantity which is capable of assuming negative as well as positive values. Whatever be the nature of the process, if it is capable of being expressed by an equation of this form, the process going on at a fixed point is called a vibration; the constant A is called the amplitude; the time 2π/n is called the period; and nt – px + a is the phase. The configuration at a given instant is called a wave, and the distance 2π/p is called the wave-length. The velocity of propagation is n/p. When we contemplate the different parts of the medium as going through the same process in succession, we use the word undulatory to denote this character of the process without in any way restricting its physical nature. A further insight into the physical nature of the process is obtained from the fact that if the two rays are polarized, and if the plane of polarization of one of them be made to turn round the axis of the ray, then when the two planes of polarization are parallel the phenomena of interference appear as above described. As the plane turns round, the dark and light bands become less distinct, and when the planes of polarization are at right angles, the illumination of the screen becomes uniform, and no trace of interference can be discovered. Hence the physical process involved in the propagation of light must not only be a directed quantity or vector capable of having its direction reversed, but this vector must be at right angles to the ray, and either in the plane of polarization or perpendicular to it. Fresnel supposed it to be a displacement of the medium perpendicular to the plane of polarization. Maccullagh and Neumann supposed it to be a displacement in the plane of polarization. The comparison of these two theories must be deferred till we come to the phenomena of dense media. The process may, however, be an electromagnetic one, and as in this case the electric displacement and the magnetic disturbance are perpendicular to each other, either of these may be supposed to be in the plane of polarization. All that has been said with respect to the radiations which affect our eyes, and which we call light, applies also to those radiations which do not produce a luminous impression on our eyes, for the phenomena of interference have been observed, and the wave-lengths measured, in the case of radiations, which can be detected only by their heating or by their chemical effects. Elasticity, tenacity, and density of the aether. Having so far determined the geometrical character of the process, we must now turn our attention to the medium in which it takes place. We may use the term aether to denote this medium, whatever it may be. In the first place, it is capable of transmitting energy. The radiations which it transmits are able not only to act on our senses, which of itself is evidence of work done, but to heat bodies which absorb them ; and by measuring the heat communicated to such bodies, the energy of the radiation may be calculated. In the next place this energy is not transmitted instantaneously from the radiating body to the absorbing body, but exists for a certain time in the medium. If we adopt either Fresnel's or Maccullagh's form of the undulatory theory, half of this energy is in the form of potential energy, due to the distortion of elementary portions of the medium, and half in the form of kinetic energy, due to the motion of the medium. We must therefore regard the aether as possessing elasticity similar to that of a solid body, and also as having a finite density. If we take Pouillet's estimate of 1.7633 as the number of grammecentigrade units of heat produced by direct sunlight falling on a square centimetre in a minute, this is equivalent to 1.234×106 ergs per second. Dividing this by 3.004×1010, the velocity of light in centimetres per second, we get for the energy in a cubic centimetre 4.1×10–5 ergs. Near the sun the energy in a cubic centimetre would be about 46,000 times this, or 1.886 ergs. If we further assume, with Sir W. Thomson, that the amplitude is not more than one hundredth of the wave-length, we have Ap = 2π/100, or about 1/16; so that we have — Energy per cubic centimetre = (1/2)ρV2A2p2 = 1.886 ergs. Greatest tangential stress per square centimetre = ρV2Ap = 30.176 dynes. Coefficient of rigidity of ether = ρV2 = 842.8 Density of aether = ρ = 9.36 ×10–19 The coefficient of rigidity of steel is about 8 ×1011, and that of glass 2.4 ×1011. If the temperature of the atmosphere were everywhere 0°C, and if it were in equilibrium about the earth supposed at rest, its density at an infinite distance from the earth would be 3 ×10–346 which is about 1.8 ×10327 times less than the estimated density of the aether. In the regions of interplanetary space the density of the aether is therefore very great compared with that of the attenuated atmosphere of interplanetary space, but the whole mass of aether within a sphere whose radius is that of the most distant planet is very small compared with that of the planets themselves.[1] The aether distinct from gross matter. When light travels through the atmosphere it is manifest that the medium through which the light is propagated is not the air itself, for in the first place the air cannot transmit transverse vibrations, and the normal vibrations which the air does transmit travel about a million times slower than light. Solid transparent bodies, such as glass and crystals, are no doubt capable of transmitting transverse vibrations, but the velocity of transmission is still hundreds of thousand times less than that with which light is transmitted through these bodies. We are therefore obliged to suppose that the medium through which light is propagated is something distinct from the transparent medium known to us, though it interpenetrates all transparent bodies and probably opaque bodies too. The velocity of light, however, is different in different transparent media, and we must therefore suppose that these media take some part in the process, and that their particles are vibrating as well as those of the aether, but the energy of the vibrations of the gross particles must be very much smaller than that of the aether, for otherwise a much larger proportion of the incident light would be reflected when a ray passes from vacuum to glass or from glass to vacuum than we find to be the case. Relative motion of the aether. We must therefore consider the aether within dense bodies as somewhat loosely connected with the dense bodies, and we have next to inquire whether, when these dense bodies are in motion through the great ocean of aether, they carry along with them the aether they contain, or whether the aether passes through them as the water of the sea passes through the meshes of a net when it is towed along by a boat. If it were possible to determine the velocity of light by observing the time it takes to travel between one station and another on the earth's surface, we might, by comparing the observed velocities in opposite directions, determine the velocity of the aether with respect to these terrestrial stations. All methods, however, by which it is practicable to determine the velocity of light from terrestrial experiments depend on the measurement of the time required for the double journey from one station to the other and back, again, and the increase of this time on account of a relative velocity of the aether equal to that of the earth in its orbit would be only about one hundred milliontth part of the whole time of transmission, and would therefore be quite insensible. The theory of the motion of the aether is hardly sufficiently developed to enable us to form a strict mathematical theory of the aberration of light, taking into account the motion of the aether. Professor Stokes, however, has shown that, on a very probable hypothesis with respect to the motion of the aether, the amount of aberration would not be sensibly affected by that motion. The only practicable method of determining directly the relative velocity of the aether with respect to the solar system is to compare the values of the velocity of light {571} deduced from the observation of the eclipses of Jupiter's satellites when Jupiter is seen from the earth at nearly opposite points of the ecliptic. [1] See Sir W. Thomson, Trans. R. S. Edin. Vol. xxi. p. 60.