The Michelson Morley Experiment
Table of Contents
The Doppler Effect
The main consequence of the finite speed of light in astronomy is that the motion of a light source relative to an observer modifies the period of the disturbance received by him.
This was enunciated in 1842 by Christian Doppler.
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
is the speed of separation of the source and observerc
is the speed of propagation of the disturbance.
If for example, the speed of separation is equal to the orbital speed 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, is about 1/10 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]
Fizeau:
- suggested a different experiment in 1852
- said 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 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 [19] another possible effect.
Fizeau investigated a beam of polarized light sent obliquely through a glass plate and the azimuth of polarization is altered based on the refractive index of the glass.
He did this with sunlight sent through the glass in the direction of the terrestrial motion, and in the opposite direction.
- The readings differed in the 2 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.
The result was negative.
Mascart stated 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.
Michelson
In 1881, however, A. A. Michelson 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.
He produced interference-fringes between 2 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 in which the aether in the neighbourhood of the earth is supposed to be set in motion.
Lorentz, however:
- showed that the quantity to be measured had only half the value supposed by Michelson.
- 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
- This was because 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 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 strongly magnetized the moving matter (iron in this experiment), so that the light was propagated across a moving magnetic field.
He 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 solved the apparent contradictions from experiments with moving bodies by extended the domain of electrical science.
H. A. Lorentz began this 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 to be in a magnetic field.
This field:
- made the forces proportional to their velocities
- communicated these forces to the ponderable matter they were associated with.
Physicists assumed that all electric and magnetic phenomena are due to the presence or motion of individual electric charges.
The discoveries of Hertz had shown that a molecule which is emitting light must contain some system resembling a Hertzian vibrator.*
Superphysics Note
The essential process in a Hertzian vibrator is the oscillation of electricity to and fro.
Lorentz himself from the outset of his career 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 main difference in Lorentz theory from those of Weber, Riemann, and Clausius is in the propagation of influence from one electron to another.
In his older writings, the electrons acted on each other at a distance, with forces depending on their:
- charges
- mutual distances
- 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.*
Superphysics Note
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.