Christian Doppler

Among the consequences of the finite speed of light which are of importance in astronomy, a leading place must be assigned to the principle enunciated in 1842 by Christian Doppler,[14] that the motion of a source of light relative to an observer modifies the period of the disturbance which is received by him.

The phenomenon resembles the depression of the pitch of a note when the source of sound is receding from the observer.

In either case, the period of the vibrations perceived by the observer is (c + v)/c × the natural period, where v denotes the velocity of separation of the source and observer, and c denotes the velocity of propagation of the disturbance.

If, e.g., the velocity of separation is equal to the orbital velocity of the earth, the D lines of sodium in the spectrum of the source will be displaced towards the red, as compared with lines derived from a terrestrial sodium flame, bs about one-tenth of the distance between them.

The application of this principle to the determination of the relative velocity of stars in the line of sight, which has proved of great service in astrophysical research, was suggested by Fizeau in 1848.[15]

An experiment of a different type was suggested in 1852 by Fizeau,[17] who remarked that, unless the aether is carried along by the earth, the radiation emitted by a terrestrial source should have different intensities in different directions.

It was, however, shown long afterwards by Lorentz[18] that such an experiment would not be expected on theoretical grounds to yield a positive result; the amount of radiant energy imparted to an absorbing body is independent of the earth’s motion.

A few years later Fizeau investigated[19] another possible effect.

If a beam of polarized light is sent obliquely through a glass plate, the azimuth of polarization is altered to an extent which depends, amongst other things, on the refractive index of the glass.

Fizeau performed this experiment with sunlight, the light being sent through the glass in the direction of the terrestrial motion, and in the opposite direction; the readings seemed to differ in the two cases, but on account of experimental difficulties the result was indecisive.

Some years later, the effect of the earth’s motion on the rotation of the plane of polarization of light propagated along the axis of a quartz crystal was investigated by Mascart.[20]

The result was negative, Mascart stating that the rotation could not have been altered by more than the (1/40,000)th part when the orientation of the apparatus was reversed from that of the terrestrial motion to the opposite direction. This was afterwards confirmed by Lord Rayleigh,[21] who found that the alteration, if it existed, could not amount to (1/100,000)th part.

In terrestrial methods of determining the velocity of light the ray is made to retrace its path, so that any velocity which the earth might possess with respect to the luminiferous medium would affect the time of the double passage only by an amount proportional to the square of the constant of aberration.[22]

In 1881, however, A. A. Michelson[23] remarked that the effect, though of the second order, should be manifested by a measurable difference between the times for rays describing equal paths parallel and perpendicular respectively to the direction of the earth’s motion.

Ho produced interference-fringes between two pencils of light which had traversed paths perpendicular to each other; but when the apparatus was rotated through a right angle, so that the difference would be reversed, the expected displacement of the fringes could not be perceived.

This result was regarded by Michelson himself as a vindication of Stokes’s theory,[24] in which the aether in the neighbourhood of the earth is supposed to be set in motion.

Lorentz[25], however, showed that the quantity to be measured had only half the value supposed by Michelson, and suggested that the negative result of the experiment might be explained by that combination of Fresnel’s and Stokes’s theories which was developed in his own memoir[26]; since, if the velocity of the aether near the earth were (say) half the earth’s velocity, the displacement of Michelson’s fringes would be insensible.

In 1897 Michelson[27] tried to determine by experiment whether the relative motion of earth and aether varies with the vertical height above the Earth.

No result, however, could be obtained to indicate that the velocity of light depends on the distance from the centre of the earth.

Michelson concluded that if there were no choice but between the theories of Fresnel and Stokes, it would be necessary to adopt the latter, and to suppose that the earth’s influence on the aether extends [errata 1] to many thousand kilometres above its surface.

The perplexity of the subject was increased by experimental results which pointed in the opposite direction to that of Michelson.

In 1892, Sir Oliver Lodge[28] observed the interference between the 2 portions of a bifurcated beam of light, which were made to travel in opposite directions round a closed path in the space between two rapidly rotating steel disks.

These showed that the speed of light is not affected by the motion of adjacent matter to the extent of (1/200)th part of the velocity of the matter.

Continuing his investigations, Lodge[29] strongly magnetized the moving matter (iron in this experiment), so that the light was propagated across a moving magnetic field; and electrified it so that the path of the beams lay in a moving electrostatic field; but in no case was the velocity of the light appreciably affected.

Theoretical physicists arrived at a solution of the apparent contradictions furnished by experiments with moving bodies and extended the domain of electrical science. This enlarged the boundaries of space and time to contain it.

The first memoir in which the new conceptions were unfolded was published by H. A. Lorentz[30] in 1892.

The theory of Lorentz was, like those of Weber, Riemann, and Clausius,[31] a theory of electrons.

All electrodynamical phenomena were ascribed to the agency of moving electric charges, which were supposed in a magnetic field to experience forces proportional to their velocities, and to communicate these forces to the ponderable matter with which they might be associated.[32]

Physicists assumed that all electric and magnetic phenomena are due to the presence or motion of individual electric charges.

Moreover, the discoveries of Hertz[34] had shown that a molecule which is emitting light must contain some system resembling a Hertzian vibrator.

The essential process in a Hertzian vibrator is the oscillation of electricity to and fro.

Lorentz himself from the outset of his career[35] had supposed the interaction of ponderable matter with the electric field to be effected by the agency of electric charges associated with the material atoms.

The principal difference by which the theory now advanced by Lorentz is distinguished from the theories of Weber, Riemann, and Clausius, and from Lorentz’ own earlier work, lies in the conception which is entertained of the propagation of influence from one electron to another.

In the older writings, the electrons were assumed to be capable of acting on each other at a distance, with forces depending on their charges, mutual distances, and velocities

In the present memoir, on the other hand, the electrons were supposed to interact not directly with each other, but with the medium in which they were embedded.

To this medium were ascribed the properties characteristic of the aether in Maxwell’s theory.

The only respect in which Lorentz’ medium differed from Maxwell’s was in regard to the effects of the motion of bodies.